Parameter estimation for reactive multi-species solute transport models is a challenging problem given the number of model parameters required and the non-linear nature of reaction terms. To address this problem, the general-purpose parameter estimation software tool PEST was applied for inverse modeling of chlorinated ethenes in ground-water systems. In this study, numerical models were developed to simulate groundwater flow (MODFLOW-2000) and the transport and biotransformation of chlorinated ethenes (SEAM3D). An optimization method was developed and tested using synthetic data. Tests performed to study the influence of the degradation kinetic state of the plume on the quality of parameter estimate using a tetrachloroethene (PCE) source demonstrated a satisfactory performance in simulating the parent compound and daughter products of reductive dechlorination. Tests involving noisy observation data showed the method was less effective for simulating vinyl chloride (VC), possibly due to cumulative error in the sequential optimization process.