Abstract: The photodegradation of phenol was studied in a batch reactor configuration illuminated with a 400 W medium pressure mercury lamp. The effects of parameters such as pH, kinetic constants and initial phenol concentration on the photolytic degradation and toxicity assay have been studied. The experimental results have shown that lower pH and lower concentration of phenol favor the phenol degradation. The disappearance of phenol in each case approximately obeyed first-order kinetics with the apparent first-order decay constant increasing with decreasing solute concentration. Bioassay tests showed that phenol were toxic to D. magna and so resulted in quite low LC50 values. Comparison of Toxicity Units (TU) between phenol and effluent toxicity has shown that TU value for effluent was 2.18 times lower than that obtain to phenol. Thus, photolysis able to eliminate the toxicity of by-products formed during the degradation of phenol.