Few studies have assessed the outdoor thermal comfort of users performing physical activity. The purpose of this research was to assess the application and performance of the COMFA outdoor thermal comfort model on subjects performing physical activity. Field tests were conducted under clear and overcast skies to evaluate the application of a cylindrical radiation thermometer, net radiometer, and a theoretical estimation model to predict the radiation absorbed by a cylindrical body ('R'). During the sampling periods, the differences between the methods used to estimate ' R' ranged from 3-8%. To evaluate the performance of the COMFA model, field tests were conducted on 27 subjects, performing 30 minutes of steady-state physical activity (walking, cycling and running) in an outdoor environment. The predicted COMFA budget values were compared to the actual thermal sensation (ATS) votes provided by the participants during each 5-minute interval. Although there was some discrepancy between the budget values and the ATS votes, the results revealed a significant positive correlation between the ATS and predicted budget scores (Spearman's rho =0.502-0.623, P < 0.01). This initial assessment revealed that the model was not accurate when subjects were performing at high metabolic rates (> 400 W m-2), high wind speeds (> 4 m s-1), and warm air temperatures (>28° C). Revisions to the clothing resistance, clothing vapour resistance, skin tissue resistance, and skin temperature equations of the COMFA model were proposed. The revised assessment revealed a clear overlap in the budget storage values ('B') for the ATS categories -1 (slightly cool) to +1 (slightly warm), and users performing physical activity had a wide range of thermal acceptability ('B' = -20 W m-2 - +150 W m-2). The revised COMFA model predicted the ATS of users approximately 70% of the time. This study effectively integrated current empirical research related to the heat and moisture exchange within the clothing microclimate to improve the application of the COMFA model for users performing physical activity.