The influence of the various parameters which govern the kinetics (mass of catalyst, wavelength, initial concentration, temperature, radiant flux, modification of the photocatalyst) has been analyzed. The knowledge of these parameters enables one to determine the best conditions for an optimum quantum yield. The photon appears as a reactant and the light flux as an additional phase (the ’electromagnetic phase’). Several examples of polyphasic photocatalytic systems are described with up to 5 phases working concertedly, especially in the case of pollutant removal from used waters.