The main aim of this article is to present an experimental analysis of the thermal influence on the positive derivatives of the real parts of impedance parameters against frequency (PDRZ) for microwave GaN and GaAs based HEMTs. The PDRZ effect is investigated with respect to temperature by both heating and cooling the studied devices. The main findings are that both devices are affected by the PDRZ effect, although with different severity and onset frequency. Due to the extrinsic resistances response with temperature, the real parts of impedance parameters shift towards higher values, especially for the GaAs HEMT. By de-embedding appropriate values of the extrinsic capacitances, the PDRZ effect can be neutralized in both devices. As the extrinsic capacitances are thermally unresponsive, roughly the same values of the extrinsic capacitances are achieved for all the studied temperatures.