This paper explores the thermodynamic limits of solar thermal systems using absorber surfaces with high angular selectivity. It shows that theoretical considerations regarding the behavior of non-concentrating solar thermal systems are not enough in themselves to rule out their use in high-temperature applications. In fact, the paper demonstrates that the maximum theoretical thermal efficiency of a non-concentrating system may equal that of a concentrating system if the monochromatic directional emittance of its absorbing surface is sufficiently restricted. It is thus confirmed that the fundamental parameter that determines the ability of a solar-energy system to operate at high temperatures is not optical concentration, but angular selectivity of emittance. (C) 2004 Published by Elsevier Ltd.