The thermal behaviour of a plate heat pipe solar collector was analysed numerically and experimentally. The numerical model is based on energy balance equations assuming a quasi-steady state condition. The major simplification was that the temperature in the heat pipe was considered to be uniform and equal to the saturation temperature. This assumption is not far from the truth, since heat pipes are considered as isothermal devices. A small-scale solar collector, with an aperture area of about 0.1 m², was experimentally tested during the Summer season in Porto. Two types of tests were made: the first was the determination of the instantaneous efficiency curve and the second was the determination of the collector time constant, a measure of its thermal inertia. Results showed a collector optical efficiency of 64% and an overall loss coefficient of 5.5 W/(m²K), for a non-selective surface coating. There was a good agreement between numerical and experimental results.