By the method of laboratory modeling, it was found that 4-chloraniline transformation in natural water at a concentration of 10 mg/L is described by a first-order equation. The activation parameters of this reaction indicate in favor of its biochemical mechanism, and the chemical (non-enzymatic) mechanism does not play a significant role in this process. The results of IR spectroscopic examination of the decomposition products of 4-chloraniline in natural water indicate the absence of an amino group in them and the preservation of most chlorine atoms.