Summary Personal protective clothing (PPC) contributes to the thermal and physiological stress experienced by workers. This stress is a major health and safety concern, particularly for firefighters. Thermophysiological stress is associated with an increased risk of cardiovascular events, the most common cause of death among firefighters. The objective of this study was to evaluate the physiological response of firefighters when wearing PPC fitted with two moisture-barrier membranes, one with improved water vapour permeability and heat transfer properties. A new concept to ensure better air circulation in the back area of the PPC was also assessed. These membranes and the new concept are based on novel technologies with the potential to reduce the thermophysiological stress experienced by firefighters. Ten participants performed walking tests at 5 km/hour on a treadmill in a climate chamber in which the temperature (35°C) and relative humidity (50%) were controlled. The workload was adjusted through the inclination of the treadmill in order to obtain an exertion level of 30% of the participants’ maximum oxygen consumption. The five PPC conditions evaluated consisted of two barrier membranes (M1 and M2 [improved]), integrated into two garment designs (Traditional and Innovative), and a new ventilation system. The heat stress generated by the different experimental conditions was assessed using the variables of oxygen consumption, internal body temperature, cardiac cost during work, fluid loss, relative humidity and the temperature inside the PPC (in the inner and outer layers), as well as the participants’ psychophysical perception of exertion. The main results suggest that there was a lower psychophysical perception of exertion with the new-generation barrier membrane (M2) than with the other membrane, for the last 20 minutes of the test. The design modifications of the Innovative model did not reduce physiological stress during exercise. In fact, relative humidity in the inner layer of the Innovative model was not statistically different than that of the Traditional model, and temperature in the inner layer of the Innovative model was significantly higher than that of the Traditional model. The Innovative model caused neck and hip discomfort during torso flexion. When a self-contained breathing apparatus was used, the new ventilation system in the back area did not reduce thermophysiological stress in a 45-minute situation of prolonged exertion. The results of this research demonstrate the importance of improving the effectiveness of the materials used in these PPCs. In fact, despite the relatively low physical effort caused by the slope and speed of the treadmill, none of the conditions evaluated led to stabilization of the variables associated with the thermoregulation process. Internal body temperature and heart rate increased progressively throughout the test. The constant increase of these two physiological variables was caused by the relative humidity inside the PPC, which rose to over 80% after a few minutes of exertion. The presence of a microclimate, in which air containing 80% of the maximum water vapour it could absorb, greatly limits the evaporation of sweat from the skin’s surface. To succeed in reducing thermophysiological stress, it seems imperative to develop materials capable of lowering the relative humidity inside PPCs.