Previously published stable isotopic data on benthic foraminiferal species from a Holocene sequence in the Celtic Sea have been interpreted in terms of the progressive replacement of a tidally mixed by a stratified water mass. Offsets in the δ18O data between Ammonia batavus and Quinqueloculina seminulum were attributed to a ‘seasonal effect’ in which these two species were hypothesized to have calcified at different times of the year. The aims of this study were to test the hypotheses (1) that benthic foraminiferal stable isotope records from across the Celtic Sea front reflect seasonal stratification and (2) that offsets in the oxygen isotope record between different species are the result of the postulated seasonal effect. Hypothesis 1 was tested through investigation of live and dead benthic foraminiferal and bottom-water δ18O and δ13C sampled in transects across the Celtic Sea front from mixed through frontal to stratified water masses. Measurements of bottom-water salinity enabled a mixing-line equation to be developed for this area enabling quantitative reconstructions of bottom-water temperature from the isotopic data. Samples from stratified settings are characterized by heavier δ18Oforam and lighter δ13Cforam values than the mixed samples. Offsets in δ18Oforam between A. batavus and Q. seminum support the notion of the seasonal effect. A. batavus produces values close to equilibrium while Q. seminulum overestimates temperature by up to 2°C and this might explain some of the offset observed between the two species observed in the palaeodata. Comparison of the δ18Ofoarm data with measured seasonal temperature cycles from mixed and stratified localities in the Celtic Sea demonstrates that, while most foraminifera calcify during the summer months, different species calcify at, or are preserved from, different times within this warm part of the seasonal cycle; Q. seminulum calcifies during September when peak bottom-water temperatures occur, while A. batavus calcifies during September in stratified localities, but during spring or early summer in mixed localities. This study confirms the interpretation of the δ18O palaeodata from the Celtic Sea as a palaeostratification record and demonstrates that δ18O data from shelf-sea cores can be used to supplement benthic foraminiferal assemblages as a tool for reconstructing the long-term dynamics of seasonal stratification.