The available solar flux at a given altitude in the atmospheres of Mars and Venus is attenuated mainly by CO 2 (molecular absorption and Rayleigh scattering) with an extra contribution due to SO 2 on Venus. The dissociation cross section of CO 2 depends on temperature. At temperatures appropriate for these atmospheres (∼250°K), the cross sections are about 15% lower than those at room conditions ( Y.L. Yung and W.B. De More, 1982, Icarus, 51, 199). It is shown that this temperature effect cannot be neglected in the evaluation of photolysis rates. Calculations of the photodissociation coefficients of CO 2, SO 2, HCl, and H 2O are presented. For example, at the surface of Mars, the coefficient of H 2O is nearly multiplied by a factor of 10!