The effects of dynamic self-heating in SOI MOSFETs are well known. For circuit simulation, it is attractive to model these effects using a simple first order electro-thermal model, but the validity of such models has not been verified previously. In this paper, direct measurements are presented of the small signal drain conductance of SOI MOSFETs for frequencies from DC up to 300 MHz. The conductance measurements clearly show a single thermal time constant. Furthermore, the thermal resistance extracted directly from small signal drain conductance data has been compared with the value obtained from gate resistance thermometry. The good agreement between both methods confirms that self-heating is dominated by a single time constant of the order of 1 Â¿s. It can be concluded that dynamic self-heating in SOI MOSFETs may be adequately modelled using a simple first order thermal model with a single thermal resistance and capacitance.