Fifteen non-aminoazo dyes were oxidized when catalyzed by horseradish peroxidase (HRP) in the presence of H2O2. Aromaticdiazonium ions, the ultimate carcinogen, were detected in the reaction with an of aza compounds as the substrates for HRP. The rates of oxidative cleavage fairly depended on pH. Relative enzymatic activities were compared among 15 non-aminoazo, dyes, and the structural requirements of the substrates for HRP were discussed in detail.