Novel experimental techniques were developed to measure changes in the weight, thickness, density, heat capacity, heat conductivity, and infrared (IR) transmission of protective fabrics occurring during a thermal protective performance (TPP) test. Comparisons are made between polybenzimidazole (PBI), aramid, a PBI/aramid blend fabric, and flame-retardant (FR) cotton fabrics in the 250 g/m² (7.5-oz/yd²) weight range. This research analyzes changes in fabric heat transfer properties produced through mechanisms of pyrolysis, char formation, and shrinkage. Fiber character is shown to play a decisive role in determining the direction and extent of change in thermophysical properties. Retention of air volume is found to be critical to prolonged thermal protection performance. Experimental data indicate that air and fiber conduction dominate in intense exposures to a mixture of radiant and convective thermal energy; direct radiant transmission is not an important contributor to the total heat transferred in these exposures. The ability of fabrics to maintain surface fibers is thought to have significant impact in blocking convective heat transmission. The degradation behaviors of different materials are compared and related to their thermal protective performance.