The absorption of solar ultraviolet radiation in the wavelength range 2000 to 2200 A by CO2 strongly reduces the dissociation rate of HCl on Venus. The Cl catalytic reaction for the rapid recombination of O and CO and the yellow coloration of the Venus haze by OCl(-) and Cl3(-) appears to be unlikely. At the time of the Martian dust storm, the dissociation of H2O in the vicinity of the surface may vanish. The increase of dissociation at high altitudes, however, can be the source of H atoms in the upper atmosphere. The H atoms from the dissociation of hydrides may catalyze the recombination of CO and O in the lower atmospheres of these planets according to the following scheme: H + CO + M yields HCO + M, and HCO + O yields H + CO2. In the case of Venus, there may be other kinds of sinks for the photodissociation products, and its chemical environment might be more complex than that of Mars.