Chemical analysis of groundwater and sediments was carried out to investigate causes of elevated F(-) (1.5-6.6 mg/L) and As concentrations (10-27 mu g/L; one sample affected by local contamination with 4870 mu g/L As), in groundwater from the Yuncheng Basin, northern China. Groundwater from 9 out of 73 wells contains both F and As concentrations above World Health Organisation safe drinking guidelines (>1.5 mg/L and >10 mu g/L respectively); F(-) concentrations above safe levels are more widespread than As (27 vs. 12 wells). The elevated As and F concentrations represent a significant health risk, as groundwater is widely used to supply agricultural and domestic water in the region. High F and As concentrations occur in shallow groundwater affected by agriculture and deep groundwater with long residence times (>13 ka) that shows little sign of anthropogenic influence. The strong positive correlation between groundwater F/Cl and As/Cl ratios (r(2) = 0.98 and 0.77 in shallow and deep groundwater, respectively) indicates that these elements are mobilized and enriched by common processes. Positive correlations between F and As concentrations and Na/Ca ratios (r2 = 0.67 and 0.46, respectively) indicate that groundwater major ion chemistry plays a significant role in mobilizing F and As. Mobilization likely occurs via de-sorption of As and F anions (e.g. HAsO(4)(2-) and F(-)) from hydrous metal oxides. Moderate positive correlations between pH and As and F concentrations (r(2) = 0.36 and 0.17, respectively) indicate that high pH may favour de-sorption, while HCO(3) may act as a sorption competitor. High groundwater Na/Ca ratios likely result from cation exchange, while pH and HCO(3) contents are predominantly controlled by carbonate weathering reactions. Sediments from the area were reacted with various water solutions, producing F concentrations between 0.49 and 2.7 mg/L and As concentrations between 0.51 and 16.7 mu g/L Up to 45 more F and 35 more As were released when sediments were reacted with a Na-rich, Ca-poor solution compared to a Ca-rich solution; this is consistent with increased mobilization of F- and HAsO(4)(2-) by Na-rich, Ca-poor groundwater. Increasing F and As concentrations across a wide area caused by high levels of pumping is a potential future health concern.