We present the results of an experimental investigation of the ultraviolet photolysis of mixtures of phosphine in hydrogen. Under no circumstances could red phosphorus be produced in this system. Rather, the product was always yellow. The measured indices of refraction of the phosphorus produced were used to fit the variation of reflectivity of Jupiter's Great Red Spot with wavelength. It was found that the phosphorus particles have to be small (0.05 μm) and their layer should have an optical depth of about one at λ = 0.4 μm.