The photochemistry of CO2 on Venus is discussed in the light of recent data obtained from Mariner 5 and Venus 4. It is suggested that preliminary data indicate that recombination of CO and O proceeds rapidly by formation of an unstable CO3 complex. Models for the thermal structure of the Venus atmosphere are presented. These models are characterized by a remarkably small temperature lapse rate, about 2°K km−1, in the stratosphere. The scale height is approximately constant and equal to 5 km over an altitude range in excess of 50 km. The exospheric temperature is of the order of 700°K. It is suggested that dissociative recombination of CO2+ could provide an important source of airglow emission in the fourth positive bands of CO. This emission system could account for the weak unidentified ultraviolet airglow detected by Mariner 5. The possibility of an important proton component of the Venus ionosphere is discussed. In particular, it is suggested that diffusion of proton from the sunlit hemisphere may provide an important nighttime source of ionization on Venus.