The commercially available substance montmorillonite KSF, used as a photocatalyst in heterogeneous systems for degradation of phenol in water, was investigated in this study. Phenol can be photodegradated in montmorillonite KSF suspension under a 250W metal halide lamp (365nm). The photodegradation was dependent on the pH of the solution, and it was more effective for phenol to be degraded at pH 4. Results also indicated that the concentrations of montmorillonite KSF and phenol could affect the degradation rate. The degradation rate of phenol increased with the increasing concentration of clays in aqueous suspensions in the range of 0.25 to 2.0wt.%, and the initial photodegradation rate of phenol increased with increasing initial concentration of phenol. Furthermore, the removal efficiency of phenol can be enhanced in the presence of carboxylic salts. A preliminary mechanism of the phenol degradation in KSF suspension is suggested that hydroxyl radicals might be photogenerated from water on the surface of KSF, and iron from KSF acts as a photocatalyst by generating hydroxyl radicals.