The effect of a model humic constituent, protocatechuic acid, on the enzymatic transformation of 3, 4-dichloroaniline (DCA) catalyzed by horseradish peroxidase (HRP) was studied. The presence of protocatechuic acid caused an increase in the rate of DCA transformation, resulting in an 80% transformation after 15 min of incubation, compared with 50% when DCA was incubated alone.<BR>The polymeric reaction products were analyzed by pyrolysis-GC/MS. More than 60% of the pyrolysis products from the DCA alone reaction product were tetrachlorodiazobenzene while this compound was not detected at all in pyrolysis products of DCA in the presence of protocatechuic acid. The major pyrolysis product in the latter case was DCA itself, which appears to have been cleaved from anilinoquinone or enaminone subunits.<BR>In addition to these major pyrolysis products, some minor peaks i.e. formanilide, diphenylamine, phenylpyrrole and isocyanobenzene were also detected. These were consistent with NMR data on the same product and are thought to represent major subunits that are resistant to cleavage from the polymer by pyrolysis.