The temperature response of field-effect transistors (FETs) to instantaneous power dissipation has been shown to be significant at high frequencies, even though the self-heating process has a very slow time constant. This affects intermodulation at high frequencies. A major difficulty in characterizing the self-heating process in microwave FETs is to differentiate between the self-heating and charge-trapping rates. An iso-trapping measurement technique is proposed by which it becomes possible to characterize the self-heating process in an FET in isolation from the effect of the charge-trapping process in the FET. The results of iso-trapping measurements performed on a GaN high-electron-mobility transistor are presented, and used to successfully characterize the self-heating process.