Core monitoring data collected from river catchments as part of the Land Ocean Interaction Study (LOIS). The Land Ocean Interaction Study was a Natural Environment Research Council thematic programme which ran from 1992 to 2000 and involved over 360 scientists from 11 research institutes and 27 universities. The LOIS project was designed to study coastal processes and investigations covered river catchments, estuaries and coastal seas and their long-term evolution, air-sea interaction and shelf-edge interactions with the open ocean. The river catchment core monitoring includes the following data available for download: continuous in situ measurements of pH, conductivity, dissolved oxygen and temperature; on site measurements of pH, conductivity and temperature; major ions and nutrients; herbicides; polychlorinated biphenyls (PCBs), chlorobenzenes (CB) and organochlorine (OC) pesticides; triazine, organophosphate and pyrethroid (TOP) pesticides; trace elements; and total alkalinity. Supporting documentation contains a list of all determinands measured and a list of sites, their locations and the frequency of sampling for each set of measurements. A subset of the LOIS chemistry, including uncensored values (i.e. instrument measurements below the stated limits of detection) are available within the UK Lowland River Chemistry dataset, available from EIDC.