The phosphine photochemistry on Saturn is studied with a 1D photochemical model. The PH3 concentration is rapidly depleted with height (scale height 3.5 km) in the upper troposphere. Formation of P, a probable precursor of P4, (a potential red chromophore in the atmosphere), is highly improbable unless the rate constant for the recombination reaction PH+H2+M→PH3+M is less than 10-41 cm6 molec-2 sec-1. Coupling of PH3 and hydrocarbon photochemistry, specifically the C2H2 catalyzed photodissociation of CH4, is important. Column production rates of the organophosphorus compounds CH3PH2 and HCP of ~3¿108 cm-2 sec-1 are predicted, with potentially observable column densities of >1¿1017 cm-2.