This is a long-term monitoring dataset of surface temperature, surface oxygen, water clarity, water chemistry and phytoplankton chlorophyll a from weekly or fortnightly sampling at the South Basin of Windermere in Cumbria, England that began in 1945 for some variables. The data were initially collected by the Freshwater Biological Association but have been collected by the Centre for Ecology & Hydrology (CEH) and its predecessor Institute of Freshwater Ecology (IFE) since 1989. The data available to download comprise surface temperature (TEMP) in degree Celsius, surface oxygen saturation (OXYG) in % air-saturation, Secchi depth (SECC) in metres, alkalinity (ALKA) in µg per litre as CaCO3 and pH. Ammonium (NH4N), nitrate (NO3N), soluble reactive phosphate (PO4P), total phosphorus (TOTP), dissolved reactive silicon expressed as SiO2 (SIO2) and phytoplankton chlorophyll a (TOCA) are all given in µg per litre. Water samples are based on a sample integrated from 0 to 5 m (1945-1962), 0 to 10m (1962-1964) and 0 to 7m (1964 onwards). Measurements are made from a boat at a marked location (buoy) at the deepest part of the lake. All data are from March 1945 until the end of 2013. Full details about this dataset can be found at https://doi.org/10.5285/e3c4d368-215d-49b2-8e12-74c99c4c3a9d