AbstractGas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS) is the preferred method of confirming the administration of exogenous testosterone by athletes. This relies on synthetic testosterone preparations being depleted in 13C compared to natural testosterone. There is concern, however, about the existence of synthetic testosterone products that are unexpectedly 13C‐enriched and which may allow athletes to circumvent the current GC‐C‐IRMS test. Further to the reported studies of legitimate pharmaceutical‐grade testosterone products, a detailed analysis of seized materials from border‐level seizures was required to obtain intelligence concerning trends in ‘black market’ testosterone manufacture and distribution. The sample set collected for this study between 2006 and 2009 inclusive provided a δ13C range (n = 266) of − 22.9‰ to − 32.6‰ with mean and median values of − 28.4‰ and − 28.6‰, respectively. Within this distribution there were 24 samples (9%) confirmed to have δ13C values in the range reported for endogenous urinary steroid metabolites (≥− 25.8‰). The benefit of δ13C profiling for testosterone preparations was demonstrated by the ability to identify specific seized products that can be target tested for future intelligence purposes.In addition, the potential of stable hydrogen isotope ratio (2H/1H; δ2H) discrimination to complement δ13C analysis was investigated. Methodologies for the determination of δ2H values by gas chromatography‐thermal conversion‐isotope ratio mass spectrometry (GC‐TC‐IRMS) were developed to provide a δ2H range (n = 173) of − 177‰ to − 268‰ with mean and median values of − 231‰ and − 234‰, respectively. Copyright © 2010 John Wiley & Sons, Ltd.