The effect of the concentration of the photocatalyst, the organic substrate and the light intensity is presented for the photocatalytic degradation of pentachlorophenol in the large solar plant built at the Plataforma Solar de Almeria. The degradation of pentachlorophenol is easily achieved in short residence times. The main factors affecting the total efficiency are discussed in view of a simple model for the primary events in the photocatalytic system. The derived approximated equations are able to fit the experimental data well. The optimization of the degradation and mineralization rates is strongly dependent on the organic substrate and predominantly depends on kinetic factors. The partition or competition of the substrate and its degradation products on the surface of the photocatalyst seems unimportant.