Understanding antibiotic biodegradation is important to the appreciation of their fate and removal from the environment.In this research an Isotope Ratio Mass Spectrometry (IRMS)method was developed to evaluate the extent of biodegradation of the antibiotic,sulphanilide,in contaminated groundwater.Results indicted an enrichment in δ13C of 8.44‰ from-26.56 (at the contaminant source) to-18.12‰ (300 m downfield of the source).These results confirm reductions in sulphanilamide concentrations (from 650 to 10 mg/L) across the contaminant plume to be attributable to biodegradation (56％) vs.other natural attenuation processes,such as dilution or dispersion (42％).To understand the controls on sulphanilamide degradation ex-situ microcosms assessed the influence of sulphanilamide concentration,redox conditions and an altative carbon source.Results indicated,high levels of anaerobic capacity (～50％ mineralisation) to degrade sulphanilamide under high (263 mg/L),moderate (10 mg/L) and low (0.02 mg/L) substrate concentrations.The addition of electron acceptors;nitrate and sulphate,did not significantly enhance the capacity of the groundwater to anaerobically biodegrade sulphanilamide.Interestingly,where altative carbon sources were present,the addition of nitrate and sulphate inhibited sulphanilamide biodegradation.These results suggest,under in-situ conditions,when a preferential carbon source was available for biodegradation,sulphanilamide could be acting as a nitrogen and/or sulphur source.These findings are important as they highlight sulphanilamide beingused as a carbon and a putative nitrogen and sulphur source,under prevailing ironreducing conditions.