Many applications for lasers require them to run at high power densities, and subsequent thermal effects can degrade their operation or limit their performance. The incorporation of a thin film of diamond in-between a laser diode and its heatsink can bring about improved laser operation through a reduction in operating temperature and temperature gradients. This paper describes the use of finite element techniques to determine the temperature distribution within a double-heterostructure laser diode and its heatsink. In the model, the diode is mounted p-side down on a copper heatsink with various thicknesses of diamond film being incorporated between the diode and the heatsink. The importance of using the correct dimensions for the diamond film are demonstrated. Various heatsink-diamond film configurations were experimentally constructed and the temperature of the active laser region measured by the nulling technique (monitoring the lasing wavelength as temperature changes). This temperature was compared to that predicted for the active region from the numerical model in order to verify the accuracy and limitations of the numerical model.