This dataset includes rainfall, cloud and stream hydrochemistry of the Plynlimon research catchments in Mid Wales. The data cover the period from June 2011 to March 2016. Sampling was weekly from June 2011 to December 2011 and then fortnightly from January 2012. Data for the whole period are presented for major anions and cations, pH, conductivity, alkalinity and in-situ measurements such as water temperature and stream flow. From June 2011 to March 2013 data are also presented for a range of metals. Data are presented for the whole period for six stream locations and one rainfall site. Data are also for presented for two streams, Nant Iago and Tanllwyth and one cloud site for the period from June 2011 to July 2012. Stream samples were taken using a grab technique. Rain samples were collected using bulk precipitation collectors. Cloud samples were collected using a Centre of Ecology and Hydrology (CEH) Edinburgh pattern lidded harp type passive collector. This data presented follow on from the hydrochemical data presented in the dataset Plynlimon research catchment hydrochemistry (44095e17-43b0-45d4-a781-aab4f72da025) Fieldwork was carried out by CEH members of staff trained in the fieldwork techniques required. Chemical analysis was carried out by qualified CEH chemists at laboratories at CEH Lancaster and CEH Bangor. The Plynlimon research catchments lie within the headwaters of the River Severn and the River Wye in the uplands of mid-Wales. Intensive and long-term monitoring within the catchments underpins a wealth of hydrological and hydro-chemical research. Monitoring is funded by the Centre for Ecology & Hydrology, and is ongoing since 1968.