Breakthrough of Cr(VI) (chromate), Se(VI) (selenate), and O2 (dissolved oxygen) was observed in tracer tests conducted in a shallow, sand and gravel aquifer with mildly reducing conditions. Loss of Cr, probably due to reduction of Cr(VI) to Cr(III) and irreversible sorption of Cr(III), occurred along with slight retardation of Cr(VI), owing to reversible sorption. Reduction of Se(VI) and O2 was thermodynamically feasible but did not occur, indicating conditions were unfavorable to microbial reduction. Cr(VI) reduction by constituents of aquifer sediments did not achieve local equilibrium during transport. The reduction rate was probably limited by incomplete contact between Cr(VI) transported along predominant flow paths and reductants located in regions within aquifer sediments of comparatively low permeability. Scatter in the amount of Cr reduction calculated from individual breakthrough curves at identical distances downgradient probably resulted from heterogeneities in the distribution of reductants in the sediments. Predictive modeling of the transport and fate of redox-sensitive solutes cannot be based strictly on thermodynamic considerations; knowledge of reaction rates is critical. Potentially important mass transfer rate limitations between solutes and reactants in sediments as well as heterogeneities in the distribution of redox properties in aquifers complicate determination of limiting rates for use in predictive simulations of the transport of redox-sensitive contaminants in groundwater.