A method for measuring the performance parameters of a solar thermal collector under non-steady conditions has been applied to an air heater with a truncated compound parabolic concentrator having an aperture area of 1.44 m(2) and a flat absorber with concentration ratio three. This type of collector was chosen in order to obtain high air temperatures in a tropical climate where the proportion of diffuse solar radiation is high. The parameters found were the optical efficiency, the linear and nonlinear heat loss coefficients, and the effective heat capacity. These parameters were determined individually in separate experiments, some with and some without solar radiation falling on the collector. Agreement within 2% was found between the optical efficiency measured in the experiments and the optical efficiency determined from direct measurements of the optical properties of the materials in the collector. The method can be performed all the year round in a tropical climate when steady conditions do not occur. (C) 2004 Published by Elsevier Ltd.