Xe lamp was employed to simulate the sunlight as light source to study the direct photolysis of three kinds of phenolic pollutants under different pH values. The effects of the dissociation on their photolysis were investigated, and a calculation method and a forecasting model for the photolysis rate constants under the situation of dissociation and non-dissociation of phenolic pollutants were established. The experiment results showed that the photolysis rate constant of pentachlorophenol (PCP) increased from 25.30 x 10(-4) min(-1) to 82.90 x 10(-4) min(-1) when pH value varied from 4.0 to 9.0, and that of nitrophenol (PNP) decreased from 11.90 x 10(-4) min(-1) to 3.18 x 10(-4) min(-1) Similar to the PNP, the photolysis rate constant of phenol decreased from 32.50 x 10(-4) min(-1) to 13.40 x 10(-4) min(-1) with the pH value increased from 4.0 to 11.0. From analysis of the results, a power function relationships between the total photolysis rate constants (K) and dissociation degrees (alpha) of these three phenolic compounds could be established. If alpha had been determined, the dissociation and non-dissociation species photolysis rate constants could be calculated by the tangent equation of the power function. The effect of dissociation on the photolysis should be mainly attributed to the formation of the negative oxygen ions, which led to the changes of the activities of benzene ring and substituent groups. These results will provide theoretical references to further understand environmental behaviors of phenolic compounds in natural waters.