Abstract Photolysis of $ NH_{3} $ at 185 nm in the presence of a two-fold excess of $ CH_{4} $ results in the loss of about 0.25 mole of $ CH_{4} $ per mole of $ NH_{3} $ decomposed ($ ΔCH_{4} $/$ ΔNH_{3} $). The loss arises from the abstraction of hydrogen atoms from $ CH_{4} $ by photolytically generated hot hydrogen atoms, the presence of which is established by the constancy of $ ΔCH_{4} $/$ ΔNH_{3} $ between 298 and 156 K and by the quenching of the abstraction reaction when either $ H_{2} $ or $ SF_{6} $ is added. From the latter result, it can be concluded that $ NH_{3} $ photolysis in the $ H_{2} $-abundant atmosphere of Jupiter is not responsible for the presence of the carbon compounds observed there such as ethane, acetylene, and hydrogen cyanide, but may have had a role in the early atmosphere of Titan. Photolysis of $ PH_{3} $ with a 206 nm light source gives $ P_{2} $$ H_{4} $, which in turn is converted to a red-brown solid ($ P_{4} $?). The course of the photolysis is not changed appreciably when the temperature is lowered to 157 K except that the concentration of $ P_{2} $$ H_{4} $ increases. The presence of $ H_{2} $ has no effect on the $ P_{2} $$ H_{4} $ yield. Photolysis of 9∶1 $ NH_{3} $∶$ PH_{3} $ gives a rate of decomposition of $ PH_{3} $ that is comparable with that observed by the direct photolysis of $ PH_{3} $. Comparable amounts of $ P_{2} $$ H_{4} $ and the red-brown solid are also observed. The mechanisms of these photochemical reactions together with their implications to the atmospheric chemistry of Jupiter are discussed. The structures of the compounds responsible for the wide array of colorse.g., brown, red and white, observed in the atmosphere of Jupiter have been the subject of extensive speculation. One theory suggests that these colors are due to organic materials formed by the action of either solar ultraviolet light or electric discharges on mixtures of $ CH_{4} $, $ NH_{3} $ and $ NH_{4} $HS in the Jovian atmosphere (Ponnamperuma, 1976; Khareet al., 1978). An alternative hypothesis is that the colors are due to inorganic compounds resulting from the photolysis of $ NH_{4} $HS and $ PH_{3} $ (Lewis and Prinn, 1970; Prinn and Lewis, 1975). In this paper we will summarize our experiments which were designed to test some of these hypotheses.