Photochemical reactions taking place on giant planets like Jupiter and possibly comets and the interstellar medium are the focus of this presentation. Reaction pathways are proposed for the coupled photochemistry of NH3 (ammonia) and C2H2 (acetylene) within the context of Jupiter’s atmosphere. The discussion is extended to the Great Red Spot, Extra-Solar Giant Planets, Comets and Interstellar Organic Synthesis.Reaction rates in the form of quantum yields were measured for the decomposition of reactants and the formation of products and stable intermediates: HCN (hydrogen cyanide), CH3CN (acetonitrile), CH3CH=N-N=CHCH3 (acetaldazine), CH3CH=N-NH2 (acetaldehyde hydrazone), C2H5NH2 (ethylamine), CH3NH2 (methylamine) and C2H4 (ethene) in the photolysis of NH3/C2H2 mixtures. The quantum yields obtained allowed for the formulation of a reaction mechanism that attempts to explain the observed results under varying experimental conditions. The formation of CH3CH=N-N=CHCH3 by a radical combination reaction of CH3CH=N• was shown in this work to be inconsistent with other experiments where the CH3CH=N• radical is thought to form but where no CH3CH=N-N=CHCH3 was detected. The importance of the role of H atom abstraction reactions was demonstrated and an alternative pathway for CH3CH=N-N=CHCH3 formation involving nucleophilic reaction between N2H4 and CH3CH=NH is advanced.