Significant challenges remain in developing reliable techniques to monitor in situ biodegradation. Stable carbon and oxygen isotope analyses of the contaminants, products of degradation, and electron acceptor(s) may provide robust means for monitoring the occurrence, pathways, and rates of intrinsic or enhanced in situ biodegradation. Results of a laboratory study using diesel fuel and a mixed microbial culture show that combined stable carbon isotope analyses of carbon dioxide and stable oxygen isotope analyses of molecular oxygen allow monitoring of the occurrence and pathways of degradation. The first-order rate constants for contaminant degradation (about −0.04 day-1) obtained from oxygen and contaminant concentrations are in excellent agreement with those obtained from isotopic data for oxygen (−0.04 to −0.05 day-1), indicating that oxygen isotope analyses of molecular oxygen can be used for quantifying the rate of contaminant degradation. Based on our results and a review of the published literature...