Linear alkylbenzenes (LABs) are common environmental contaminants associated with a range of industrial and domestic activities. Although natural environments receiving LABs may exhibit a range of redox conditions, until now only aerobic biodegradation of these compounds has been demonstrated. Where LAB contamination occurs, it is important to identify degradation and measure its extent in order to make decisions on whether monitored natural attenuation (intrinsic bioremediation) is sufficient, or whether active remediation techniques are required. We have demonstrated that LABs are degraded under both aerobic and nitrate-reducing conditions. Statistical analysis of these results and published data indicate that the C12 LAB isomeric ratio varies with biodegradation, independently of the terminal electron acceptor used. Biodegradation (B, %) can be estimated from the ratio of internal to external isomers of C12 LAB by the equation B = 78 x log10 (I:E) + 16.4. This relationship can be used to determine the degree of biodegradation of LABs in a range of environments, including sites where the redox history is unknown, making it a powerful yet simple tool for monitoring LAB biodegradation in the environment.