Water temperature data from selected UK Environmental Change Network (ECN) freshwater sites (lakes, rivers, streams). These data are collected at ECN's freshwater sites in accordance with the ECN Surface Water Chemistry & Quality protocol: http://www.ecn.ac.uk/measurements/freshwater/fwc-fwa. Surface water temperatures are recorded by hand-held thermometers at the freshwater chemistry sampling point when water samples are collected. The temporal range varies from site-to-site (earliest record from 1975, latest from 2012). ECN (www.ecn.ac.uk) is the UK's long-term environmental monitoring programme. It is a multi-agency programme sponsored by a consortium of fourteen government departments and agencies. These organisations contribute to the programme through funding either site monitoring and/or network co-ordination activities. They are: Agri-Food and Biosciences Institute, Biotechnology and Biological Sciences Research Council, Cyfoeth Naturiol Cymru - Natural Resources Wales, Defence Science & Technology Laboratory, Department for Environment, Food and Rural Affairs, Environment Agency, Forestry Commission, Llywodraeth Cymru - Welsh Government, Natural England, Natural Environment Research Council, Northern Ireland Environment Agency, Scottish Environment Protection Agency, Scottish Government and Scottish Natural Heritage. The organisations providing the data included in the file are: Centre for Ecology & Hydrology, Environment Agency, James Hutton Institute, ENSIS Ltd (University College London) and Scottish Environment Protection Agency.