Knowledge of the 238U concentration in seawater is important for 234Th:238U disequilibrium studies of particle fluxes. However, these concentration data are normally obtained through a standard relationship between 238U and salinity, which has been determined for the open ocean. This study examines 238U data from both the open Mediterranean and the coastal Thermaikos Gulf, Greece, and compares it to the open ocean. No deviation from the open ocean 238U – salinity relationship was found for the Thermaikos Gulf, but some enhancement was noted close to Thessaloniki in the vicinity of a phosphate fertiliser plant. The open Mediterranean data showed a small enhancement relative to the open ocean. Although an analytical bias could not be ruled out, a review of 238U and salinity data in the literature shows that the standard relationship may not be as robust as is often assumed and the 1 % uncertainty typically used is not justified at the present time. Nevertheless, salinity-based derivations continue to be the most appropriate means of determining 238U concentrations for routine applications. We propose a new relationship that accounts for the uncertainties observed, i.e. 238U (dpm l-1) = (0.0713 ± 0.0012) x salinity.