The study examined the ability of a mechanistic biofilm kinetic model to predict the rate at which natural biofilms removed trace concentrations of p-cresol and a 2,4-D ester. Natural biofilm samples were obtained by colonizing Teflon strips at a river site (a bacterial biofilm) and at two pond sites (mixed algal-bacterial biofilms). Biofilm density, biofilm thickness, and intrinsic microbial rate constants were calculated independently. Mass transport parameters were obtained from existing equations. Although the biofilm model treated the two types of biofilms as if they were homogeneous, the tested biofilm kinetic model successfully predicted the rates at which p-cresol and the 2,4-D ester were removed by natural biofilms when the experimental reactors were operated under a wide range of biological and mass transport conditions. (Copyright (c) 1988 Pergamon Journals Ltd.)