The Carnot cycle of an electron-hole gas, exchanging heat in the form of blackbody radiation with two heat reservoirs, can be easily determined and used to study the thermodynamics of photovoltaic conversion. The thermodynamic efficiency (different from the usual practical efficiency) will be maximum, equal to the Carnot factor, when the evolution of the system is reversible. The reversibility conditions are met only for a two-level system, at open circuit, with purely radiative recombination. This case is the equivalent of the ideal gas Carnot engine and yields the Carnot efficiency (although the practical efficiency is zero, since the output power is vanishingly small). All other configurations lead to a loss in thermodynamic efficiency; the practical efficiency will, in all cases, be lower than the thermodynamic one (this discussion does not apply to multispectral cells). The identity of the hot reservoir temperature with the “effective temperature”, as usually defined, can be shown.