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In whole-sediment toxicity tests employing metal-spiked sediment, pH is a crucial determinant of sediment geochemistry, sediment-porewater metal partitioning and metal bioavailability to benthic organisms. Small pH changes in metal-spiked sediment may therefore have important implications for toxicity to organisms. The present study evaluated the effect of small-scale pH changes (pH 6.6, 7.2, 7.6) in Cu-spiked sediment on metal bioavailability and subsequent organism response. Variation of pH produced distinct changes in sediment geochemistry. Increasing pH from 6.6 to 7.6 resulted in lower redox potential, greater acid volatile sulfide (AVS) formation, and lower partitioning of Cu, Fe and Mn to the porewater. The minor pH changes and associated variation in sediment geochemistry modified reburial rates and copper accumulation by the bivalve, Austriella cf plicifera. Bivalve reburial was slowest for the 1400 mu g g(-1) Cu-spiked pH 6.6 sediment, in which porewater copper concentrations exceeded 50 mu g L-1. Bivalve reburial was higher for pH 7.2 Cu-spiked sediments at all concentrations of copper investigated than in pH 6.6 and 7.6 sediments, where reburial rates were similar. Higher levels of bivalve activity suggest that A. cf plicifera may have an optimal pH range in the vicinity of pH 7.2. Tissue Cu concentrations varied with pH and appear dependent on spiked Cu concentration and organism activity. Strong linear correlations of tissue Cu concentration with all three Cu phases (overlying water, porewater and particulate phases), however, limited the ability to accurately determine the source of Cu uptake. Refereed/Peer-reviewed