We examined the biodegradability of polyethylene glycol by analyzing gel permeation chromatography patterns, which were obtained before and after cultivation of a microbial consortium E-1 in culture media containing polyethylene glycol as its sole carbon source. We solved an inverse problem of the initial value problem, which consists of an exogenous depolymerization model and the initial weight distribution, in order to determine an oxidation rate, for which the solution of the initial value problem also satisfies the final condition. Given the oxidation rate, we simulated the transition of the weight distribution solving the initial value problem. We interpret the result that reflects a physical significance of the molecular weight on the oxidation rate, and reach a conclusion concerning the role of the membrane transport in
exogenous depolymerization processes.