A method for describing the microbial degradation of xenobiotics through the use of a second-order reaction equation was tested in several water bodies in the United States and Russia. The experiment was aimed at studying the microbial transformation of a herbicide widely used in rice growing, propanil; of a promising new fungicide, metalaxyl; and of phenol and p-cresol. The method described, in a satisfactory way, the process of microbial degradation of potentially toxic chemicals undergoing hydrolytic or oxidative transformation. Prediction of the fate of persistent compounds, like metalaxyl, is difficult, however. A data bank of second-order rate constants for organic chemicals would permit the prediction of contaminant fate in any natural water source. Such predictions would be important for assessing contaminant fate and exposure in natural waters.