Summary The aging of the population is a complex and sensitive topic involving many stakeholders and systems (work, health, finances). This study explores sustainable return to work (S-RTW) among workers aged 45 and over (older workers) following an injury that is psychological (e.g. depression) or physical (e.g. musculoskeletal disorders) in nature. In Québec, an analysis of data for the years 2013-2015 from the Commission des normes, de l'équité, de la santé et de la sécurité du travail (CNESST) reveals that workers aged 45 years and over average longer sick leaves for compensated lost-time (CLT) injuries that are psychological or physical in nature than do workers aged 44 years and under (Busque and Lebeau, 2019). The total costs and average annual costs per injury were also higher for older workers (Busque and Lebeau, 2019) for the same period. The general objective of this study was to identify the determinants of S-RTW among workers aged 45 and over who had experienced a psychological or physical injury. This objective was broken down into four specific objectives: (a) to develop the tool titled “WAC45+” (acronym for Work Accommodations for older workers aged 45 years and over) in order to document the specific work accommodations that should be implemented to facilitate the S-RTW of workers in this age category who have experienced a psychological or physical injury; (b) to validate the WAC45+; (c) to identify which accommodations are implemented most frequently; and (d) to define a S-RTW prediction model including psychosocial factors, work accommodations, and the positive and negative aspects of occupational health psychology. The study had a mixed design with both a qualitative component (Phase 1) and a quantitative component (Phase 2). In Phase 1, 32 semi-structured interviews were conducted with older workers who had experienced an injury that was totally or partly work-related (n = 15, had experienced psychological injuries; n = 17, had experienced physical injuries). Analyses and interpretation of the content of the interviews highlighted the essential elements needed to develop the conceptual dimensions and statements (items) for the WAC45+. This first phase was followed by Phase 2, in which the questionnaire was validated and longitudinal data collection was performed (follow-up at six months) with 65 older workers (n = 34, had experienced psychological injuries; n = 31, had experienced physical injuries). The S-RTW was measured using the duration of S-RTW variable (time elapsed, in months, between the date of the RTW and the date at T2). Different types of statistical analyses were performed to achieve the specific objectives of the study. The work accommodations most frequently implemented (≥ 70%) were the Return to work plan and follow-up and the Work schedule and workplace accommodations. The Health in the workplace and Support from co-workers accommodations revealed differences by type of injury, as these accommodations were implemented more often to facilitate S-RTW following physical rather than psychological injuries. These accommodations were implemented more often for workers who had experienced a physical injury, conceivably because the people around them probably knew their diagnosis and/or the workers disclosed it themselves in order to ask for accommodations. Asking for accommodations was often more difficult in cases of sick leave for psychological injuries. The duration of the S-RTW was influenced by two variables related to the work context (Ergonomics) and to occupational health psychology (Fear of a relapse). Generally speaking, it was observed that the presence of accommodations and psychosocial factors conducive to an atmosphere of safety and health-work balance, as well as aspects of occupational health psychology, contributed indirectly to the longer duration of the S-RTW of the older workers. Based on the results of this study, the stakeholders in the workplaces concerned (e.g. individuals filling the role of return-to-work coordinators, direct supervisors, unions, and work rehabilitation professionals) will be able to use the WAC45+ to identify, on the one hand, accommodations that could be implemented to facilitate the S-RTW of older workers, and on the other, drivers that could help ensure older workers’ ability to stay at work safely.  In this study, sustainable return to work is synonymous with “staying at work,” meaning that the worker remained in the same organization and held the same job as prior to the sick leave or another job, with a full-time or part-time employment relationship. A return to work is “sustainable” if the worker stays at work for at least six months after returning from a sick leave for a psychological or physical work-related injury (definition adapted from Dekkers-Sánchez, Wind, Sluiter and Frings-Dresen, 2011; Etuknwa, Daniels and Eib, 2019).  In this study, work accommodations refers to the “reasonable and feasible” measures taken by the employer and other stakeholders in the organization to facilitate S-RTW. These measures may be the result of the employer’s legal obligation to provide accommodations, or may involve minor adjustments or natural supports unrelated to the legal aspect of the term (Corbière et al., 2019; Corbière, Villotti, Lecomte, et al., 2014; MacDonald-Wilson, Rogers, Massaro, Lyass and Crean, 2002; Schultz, Krupa, Rogers and Winter, 2012; Villotti et al., 2017). Work accommodations concern more the work context, work content, and interpersonal relations. However, non-work-related accommodations made by the workers themselves and by other actors in their lives outside work may also be made to facilitate S-RTW following sick leave for a work-related injury (Negrini et al., 2019).