This paper describes a new method for the mapping of local temperatures in the active region of highpower III-V semiconductor transistors for microwave applications. The measurement technique involves scanning a focused laser beam at the surface of a chip inside its package, while the photoluminescence (PL) or the Raman spectra produced are recorded sequentially for each position of the laser beam. The local temperature is deduced either from the corresponding wavelength shift of the PL (which represents changes in the band-gap due to heating) or from Raman Stokes peak shift or from the Stokes to anti-Stokes intensity ratio (which correspond to changes in optical phonon frequencies and population respectively due to heating). Results are shown both for SiC-based field effect transistors and for bipolar type transistors (heterojunction bipolar transistors – HBTs – in the GaAs/Ga1-xInxP system). A spatial resolution of 1 µm and an accuracy in the temperature determination of ± 3 °C are demonstrated, especially for the HBTs. Finally, procedures are proposed to implement the information on local operating temperatures provided by this method into thermal resistance calculations.