Cometabolic biodegradation processes are important for bioremediation of hazardous waste sites. However, these processes are not well understood and have not been modeled thoroughly. Traditional Michaelis-Menten kinetics models often are used, but toxic effects and bacterial responses to toxicity may cause changes in enzyme levels, rendering such models inappropriate. In this article, a conceptual and mathematical model of cometabolic enzyme kinetics is described. Model derivation is based on enzyme/growth-substrate/nongrowth-substrate interactions and incorporates enzyme inhibition (caused by the presence of a cometabolic compound), inactivation (resulting from toxicity of a cometabolic product), and recovery (associated with bacterial synthesis of new enzyme in response to inactivation). The mathematical model consists of a system of two, nonlinear ordinary differential equations that can be solved implicitly using numerical methods, providing estimates of model parameters. Model analysis shows that growth substrate and nongrowth substrate oxidation rates are related by a dimensionless constant. Reliability of the model solution procedure is verified by analyzing data sets, containing random error, from simulated experiments with trichloroethylene (TCE) degradation by ammonia-oxidizing bacteria under various conditions. Estimation of the recovery rate constant is determined to be sensitive to initial TCE concentration. Model assumptions are evaluated in a companion article using data from TCE degradation experiments with ammonia-oxidizing bacteria.