The sediments of the Slindon Formation at the junction of the Chalk South Downs and the West Sussex Coastal Plain, as revealed and studied at Boxgrove, contain evidence for early Middle Pleistocene environments in southern England around half a million years ago. The archaeological importance of the deposits is attested to by the recovery of stone and organic tools, butchered fauna and fossil hominin remains. We combine palaeoecological and geochemical analyses of ostracods from the coastal plain deposits to reconstruct the climate and environment at one period of the hominin occupation. The stratigraphy of our study area is indicative of a terrestrial environment surrounding a small lake or pond. Ostracod assemblages from the sediments indicate that the ponds were shallow but permanent, and fed by groundwater and springwater. However, the species alone cannot confirm that these ponds were completely fresh. Application of Mutual Climatic Range (MCR) techniques shows that summer temperatures were similar to present-day values and winter temperatures were probably colder, whereas mean annual air temperature was similar to present or perhaps slightly cooler. Trace-element and strontium-isotope analyses of ostracods from the sediments support the conclusion that the ponds were fed largely by fresh groundwater derived from the chalk, although there may have been a minor seawater input. The carbon-isotope ratios of the ostracod shells are consistent with a groundwater source, although they also indicate that the dissolved inorganic carbon most likely did not reach equilibrium with atmospheric carbon dioxide, suggesting that the ponds had short residence time. The combination of palaeotemperature estimates with oxygen-isotope values from ostracod shells allowed us to reconstruct the oxygen-isotope ratio of the Pond water, which was close to the value of early Middle Pleistocene precipitation at this site. Values were similar to present day, suggesting that there may have been a change in seasonality or precipitation source, since the mean air temperatures might have been lower. © 2009 Elsevier Ltd.