The purpose of this experiment was to determine the effect of soil loading rate on the microbial performance during active phase co-composting of diesel-fuel contaminated clay soil under simulated windrow composting conditions. Microbial performance was monitored through relative heat generation, volatile solids destruction, and headspace oxygen/methane levels. Additional analyses in the form of radio-labelled diesel fuel, which was monitored through NaOH traps for respired 14CO2, and total petroleum hydrocarbon (TPH) concentrations were attempted during the experimental run. A total of seventeen biocells were used during the experiment. Soil loadings ranged from 0% contaminated soil to 30% contaminated soil. Each biocell received the same amount of compost amendments, with altering soil loadings. Biocells were placed in an environmental chamber for a duration of two weeks. During that time, the chamber temperature was ramped to simulate temperatures within a compost heap. Biocell height and temperature readings were taken at least three times daily. Air was supplied to the biocells for five minutes every hour, and the offgas from the biocells was bubbled through NaOH traps to capture respired CO2 and 14CO2. (Abstract shortened by UMI.)