Oxygen and carbon isotope analyses have been performed at close sampling intervals on rocks from several exposures that show the boundary between the Wenlock and Ludlow Series in the classic Silurian areas of Wales, the Welsh Borderland, and the West Midlands of England. Our data indicate a monotonic decline in δ13C from positive to negative values across the boundary between the Much Wenlock Limestone Formation (Wenlock Series) and the Lower Elton Formation (Ludlow Series), and correlatives, in all sites investigated. A slightly earlier negative excursion is also present in the upper Wenlock strata of two of these localities. Both negative δ13C excursions appear to be contemporaneous with episodes of increased ocean ventilation and eustatic sea- level lowstand postulated by Kemp. At least one, and possibly both, of the late Wenlock carbon isotope depletions can also be related to the decline in pelagic graptolite diversity. The distribution of the carbon isotope depletion in both carbonate-platform and off-shelf facies implies that this trend is unlikely to have been caused by local diagenetic alteration. Isotope chemostratigraphy may also provide an extra correlation tool that can be integrated with traditional biostratigraphic methods for Silurian strata.