Wrinkles frequently appear in geomembranes exposed to solar radiation. In 1992, Giroud and Morel established a model to predict the wrinkle geometry according to the temperature and thermal characteristics of the geomembrane. The aim of this paper is to throw some light on the thermal behaviour of geomembranes subjected to thermal radiation. The overall heat exchange coefficient with the surroundings, the absorption coefficient and the coefficient of thermal expansion were measured in different conditions for several geomembranes made of polyethylene and polyvinyl chloride. The results show the effectiveness of a white coating on geomembranes as a protection against solar radiation. A finite-difference model was developed to predict the geomembrane temperature in relation to the overall heat exchange coefficient, the absorption coefficient and the meteorological conditions. Results obtained tally closely with laboratory and in-situ measurements. It appears that the proposed model could be used in conjunction with the one developed by Giroud and Morel, which was also checked on the same experimental site.