The recent discovery in St John's College of a mass burial of mostly young adult males with severe perimortem blade trauma has prompted the suggestion that these may be related to the St Brice's Day Massacre in Oxford on 13th November AD 1002. Three radiocarbon determinations suggest that a date in the tenth century is more likely. We have nevertheless undertaken an isotopic study of the bone collagen (δ13C and δ15N) and dental enamel (δ13C, δ18O and 87Sr/86Sr) in an attempt to answer the question ‘were these individuals of Danish descent?’ Our conclusion is somewhat ambiguous, but the bone collagen suggests a diet more like other Scandinavian populations than that of local groups, and the enamel isotopes point towards a Scandinavian rather than a lowland English origin. Comparison with Oxford Archaeology's recently excavated Weymouth Ridgeway mass burial suggests, however, that the execution of a captured raiding party is more likely than the slaughter of Oxford inhabitants of Danish descent.