The objective of this work has been to investigate the possibility of using fermented whey as an organic growth supplement in order to enhance the aerobic degradation of n-hexadecane in soil. Fermented whey was added at different dosages to nutrient amended soil microcosms contaminated with 5000 mg diesel fuel kg-1 dry weight (dw). The target substance was 14C-labeled n-hexadecane, and the biodegradation was monitored by analysis of evolved 14CO2. Biodegradation curves were fitted to a three-half-order kinetics model. Enhanced biodegradation was observed in sand at 7 and 22C and in loamy sand at 22C but the effect was most pronounced in the sand soil at 22C. The addition of 6 or 60 ml fermented whey kg-1 soil dw, increased the degree of n-hexadecane biodegradation at the end of the experiment, 167 days, from 49% in the untreated sand to 60 or 67% respectively. This increase in biodegradation was characterized by an increase in the amount of substrate biodegradation 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.