The photochemical reaction, whereby dissolved P 4-molecules form red phosphorus has been found to be a monomolecular reaction. It is concluded that the light absorption causing this reaction is unchanged by neighbouring P 4-molecules and that the reaction consists in a dissociation P 4 = 2P 2, the recombinating P 2-molecules forming red phosphorus. The, light absorption of P 4, uncoloured molten and red phosphorus is studied in the near ultraviolet region. Increasing absorption with shorter wavelength is found. The measurements are discussed and related with known data from molecular spectra.