To date, the δ 18 O and δ 2 H stable isotopic composition of freshwaters has received little attention compared to stable isotopes in precipitation. Specifically, studies characterising the stable isotopic variability of surface waters on regional and local scales are limited. Here, the δ 18 O and δ 2 H stable isotopic compositions of raw and partially treated waters from 16 UK water treatment works are examined. The water treatment works abstract surface waters from a range of catchments (upland versus lowland and semi-natural versus anthropogenically impacted). The investigation of stable isotope composition and deuterium excess (d-excess) revealed a high degree of spatial and temporal variability. Compared to rainfall, the spatial and temporal variability of surface water is more complex and catchment-dependent. Multiple linear regression analysis identified moderately strong relationships between catchment controls (altitude, mean rainfall, base flow index, aquifer permeability, location) and δ 2 H, δ 18 O and d-excess of raw and partially treated waters. Results suggest that catchment controls explain 60% ( δ 2 H), 66% ( δ 18 O) and 74% (d-excess) of the variation in isotopic composition of raw water. Results also indicate that catchment permeability has a stronger influence on surface waters isotopic composition than altitude, latitude or rainfall effects. The potential benefits from the use of isotopic composition data in the water supply are discussed.