A comprehensive study has been undertaken to better understand the thermal conditions within high power, radio frequency (r.f.) signal amplifiers. Here, the results of two aspects of this study are presented in detail. First, laboratory and field experiments were conducted to determine the junction temperatures of the main heat dissipating components in the amplifier circuit. With individual devices dissipating up to 130W each, under normal operating conditions, junction temperatures were shown to approach their maximum rating of 200 °C when ambient temperatures reach 60 °C. Under these conditions the interface thermal resistance between the components and the heat sink currently accounts for over 20% of the total temperature rise from the ambient air to the device junction. Experiments were subsequently conducted to determine the interface thermal resistance of a number of candidate materials allowing comparison of application specific data obtained here with the manufacturer published data. The experimental data from this study was also used to compare the performance of the current thermal grease compound with a range of commercially available alternatives. At maximum power dissipation, junction temperatures were reduced by up to 20 °C through the use of a high conductivity thermal grease in conjunction with careful preparation of the contact surfaces to ensure consistent flatness.