Here we present a comparative study of stable carbon (δ13C) and nitrogen (δ15N) isotope data from 81 individuals from the catastrophic death assemblage at Herculaneum (79 CE) and compare these with the attritional sites of Velia (Salerno, Italy, 1st–2nd century CE) and Isola Sacra (Rome, Italy, 1st–2nd century AD). The instantaneous deposition of the Herculaneum assemblage highlights some interesting differences in our contextual and methodological understanding of stable dietary isotopes, suggesting that isotopic variation between sites may sometimes be a result of greater temporal variability rather than truly comparable differences. Our results suggest that the people of Herculaneum obtained a relatively small proportion (ca. 30%) of their dietary carbon from marine foods; the majority originating from terrestrial foodstuffs of a similar carbon isotopic composition, most likely cereals. Also observed is a generally greater dietary isotopic enrichment in male individuals than females. We infer that males had greater access to fish which may be reflective, in part, of the sociodemographic framework characteristic of Roman society. Finally, we highlight the methodological challenges which may be faced when undertaking comparisons of δ13C and δ15N data between the various age-related strata of a population, particularly due to the slow and variable rate of collagen turnover.