Micro-organic herbicide levels in river water for various sites within the Humber and Tweed catchments collected as part of the Land Ocean Interaction Study project (LOIS). The dataset contains data for Phenyl urea and Phenoxy acid herbicides, measured as 2,4-Dichlorophenoxyacetic acid, 4-(4-Chloro-O-tolyloxy) butyric acid, 4-Chloro-O-tolyloxyacetic acid, Chlorotoluron dissolved, Diuron dissolved, Isoproturon dissolved, Linuron dissolved, Mecoprop dissolved. Phenyl urea herbicide data is available for twelve sites in the Humber catchment within 1994 to 1995 and 1994 to 1997 and for three sites on the Tweed catchment within 1995. Phenoxy acid herbicide data is available for six sites (S1, U3, N4, W5 and O6) over the period December 1994 to September 1995, for 7 sites (S2, D7, A8, C9, D10, T11 and O12) over the period December 1994 to February 1997 and 3 sites (TW13, TW14 and TW15) over the period January 1995 to September 1995. Attempts were made to sample the sites at weekly intervals. However sampling was halted for short periods when it was not possible to process the samples quickly. Linuron dissolved was only measured from April 1994 - October 1994. Samples were collected in chromic acid-washed 1 litre glass bottles. Herbicide levels were concentrated before being measured using High Performance Liquid Chromatography for Phenyl urea herbicides and Gas Chromatography for Phenoxy acid herbicides. Until November 1994, analysis was completed by the York University and the Institute for Freshwater Ecology, Wareham, laboratories. From December 1994 onwards the samples were dispatched to the Institute for Hydrology, Wallingford, for extraction and analyses. Full details about this dataset can be found at https://doi.org/10.5285/e28ffc01-880d-423f-acb6-879b9fd4603a