The ultraviolet photolysis of PH3 in the laboratory leads to the formation of elemental red phosphorus, and the occurrence of the same reactions in the atmosphere of Jupiter has been suggested as the source of the chromophore of the Great Red Spot. The inclusion of either C2H2 or C2H4 as scanvenger molecule in the laboratory photolysis at 295 ¿K completely suppresses the formation of red phosphorus. Experiments are needed on the activation energies for the reactions of PH2, PH and P2 with C2H2 processes are also effective at Jovian Atmospheric temperatures. If the scavenging reactions are effective there, the formation of red phosphorus should occur only in regions with low concentrations of C2H2 and C2H4.