Industrial organic byproducts such as whey or fermented whey contain easily accessible carbon and micronutrients which could be used to stimulate microbial degradation of hazardous organic contaminants in soil. In this work we have investigated the possibility to use fermented whey as a growth supplement to enhance the aerobic degradation of diesel fuel hydrocarbons in soil. Experiments were carried out with two soils, a sand soil and a loamy sand soil, contaminated with diesel fuel at a concentration of 5000 mg kg-1 soil dry weight (dw). Fermented whey was added at different dosages to nutrient amended soil microcosms. Petroleum hydrocarbon mineralization was monitored by analysis of evolved 14CO2 from added 14C-labeled n-hexadecane. Mineralization curves were fitted to a three-half-order kinetics model. Enhanced mineralization was observed in sand soil at 7 and 22C and in loamy sand soil at 22C but the stimulatory effect was most pronounced in the sand soil at 22C, where the addition of 6 and 60 ml fermented whey kg-1 soil dw, increased the final degree of n-hexadecane mineralization from 49% in the control soil to 60 and 67% respectively. The increased total mineralization was characterized by an increase in the amount of substrate mineralized by first-order kinetics despite a decrease in the first order rate constant, k1. The highest concentration of fermented whey, 60 ml kg-1, gave rise to substrate competition, diauxie, which resulted in an extended lag phase.