In the domain of heat radiation, and particularly of solar energy use, the notion of exergy (or alternatively entropy) of radiation has given rise to a fairly abundant literature; unfortunately, incoherencies and discrepancies between the various authors could lead to a complete disuse of the notion in this context. The aim of this paper is to contribute to a clarification of this issue. We propose here a derivation of the exergy of radiation, based solely on classical thermodynamics notions, making thus possible a ready check of the validity of the results. Results are given first for the blackbody case, then extended to a radiation with arbitrary spectrum. Finally, application of the notion of radiation entropy to a few simple examples shows the consistency of this notion with some well-known physical laws, and can even give some insight into the real signification of these laws. (C) 2003 Published by Elsevier Ltd.