AlInP commonly serves as the window layer in high bandgap III-V solar cells where it is responsible for reducing surface recombination by reflecting minority carriers. It must be optically transparent and conductive to majority carriers, and so is typically thin, 25 nm, and doped. It is the semiconductor layer most exposed to the environment during operation, which consists of high temperature and concentrated light in a terrestrial concentrating system. In this paper, the oxidation of AlInP was studied as it relates to III-V terrestrial solar cells. The effects of heat, humidity, and light were investigated. Undoped AlInP samples in a light-soaked, damp heat condition grew more than 20 nm of oxide in 2400 hours, as compared to 3 nm of oxide when in the same damp heat condition without light. Under the light-soaked, damp heat condition, n-type material oxidized faster than p-type material. These effects are indicative of a photoelectrochemical oxidation reaction between the semiconductor and an electrolyte, which is provided by the humidity in this case. The removal of UV light by the use of UV absorbing glass reduced much of this additional oxidation. The removal of humidity and UV limited oxide growth to 1.2 nm after 700 hours exposure. Although direct exposure of AlInP caused oxidation, GaInP solar cells utilizing n-AlInP windows were directly exposed to light and damp heat for over 2800 hours and found stable, an effect attributed to differences between n-AlInP window layers and n-AlInP epilayers.