The temperature of supported MoS2 and Pt catalysts was measured during microwave heating at constant input power by inserting a thermocouple into the center of the catalyst bed immediately after the microwave power had been turned off at the end of progressively longer heating times. Thermal runaway was observed with MoS2/Al2O3 and Pt/Al2O3 at temperatures in the range 285-300 degreesC when the power absorbed rose with temperature at a rate greater than that at which heat is lost by cooling. This condition was not attained with MoS2/SiO2, and thermal runaway was not observed. For the MoS2 catalysts, the fraction of the microwave power absorbed, gamma, was found to vary with temperature according to the equation gamma = c(A) + c(M) exp(-E-a/RT), where c(A) and c(M) are constants and the activation energy, E-a, is 9.3 kJ mol(-1) for MoS2/Al2O3 and 11.1 kJ mol(-1) for MoS2/SiO2.