Measured diffusion thermopower of a low-disorder two-dimensional hole system in the extreme quantum limit is used to probe the thermal properties of the recently proposed particle-flux composite fermions (CF's). The data are consistent with the CF's exhibiting the integral quantum Hall effect away from filling factor nu=1/2. The magnitudes of the thermopower maxima between the fractional quantum Hall states yield an estimate for the CF Landau-level broadening Gamma(CF) which agrees well with Gamma(CF) deduced from the analysis of the excitation energy gaps for the fractional liquid states.