The impact of dissolved wastewater constituents on the treatment of synthetic phenol solutions using horseradish peroxidase (HRP) and hydrogen peroxide was investigated under a variety of reaction conditions. The constituents studied included various inorganic salts, organic compounds and heavy metals. Higher H2O2 doses were required to treat phenol in the presence of sodium sulfite, thiosulfate and sulfide; however, enhanced levels of phenol conversion were achieved once sufficient H2O2 was supplied. Sulfide and cyanide inhibited phenol transformation. The inhibition of sulfide was overcome by supplying sufficient H2O2 to oxidize the sulfide to sulfur. However, increasing the H2O2 dose was ineffective in attempting to overcome the strong inhibiting effect of cyanide. Among the heavy metal ions tested, only Mn(II) substantially inhibited phenol removal when it was present at a concentration of 1 mmol dm(-3). The presence of inorganic salts including NaCl, CaCl2. MgCl2, NH4Cl and (NH4)(2)SO4 reduced phenol conversion as compared with the treatment in distilled-deionized water. This can be attributed to the increased ionic strength of the solution.