O₂ reduction and denitrification rates were determined in shallow aquifers of 12 study areas representing a wide range in sedimentary environments and climatic conditions. Zero‐ and first‐order rates were determined by relating reactant or product concentrations to apparent groundwater age. O₂ reduction rates varied widely within and between sites, with zero‐order rates ranging from <3 μmol L⁻¹ yr⁻¹ to more than 140 μmol L⁻¹ yr⁻¹ and first‐order rates ranging from 0.02 to 0.27 yr⁻¹. Moderate denitrification rates (10–100 μmol N L⁻¹ yr⁻¹; 0.06–0.30 yr⁻¹) were observed in most areas with O₂ concentrations below 60 μmol L⁻¹, while higher rates (>100 μmol N L⁻¹ yr⁻¹; >0.36 yr⁻¹) occur when changes in lithology result in a sharp increase in the supply of electron donors. Denitrification lag times (i.e., groundwater travel times prior to the onset of denitrification) ranged from <20 yr to >80 yr. The availability of electron donors is indicated as the primary factor affecting O₂ reduction rates. Concentrations of dissolved organic carbon (DOC) and/or sulfate (an indicator of sulfide oxidation) were positively correlated with groundwater age at sites with high O₂ reduction rates and negatively correlated at sites with lower rates. Furthermore, electron donors from recharging DOC are not sufficient to account for appreciable O₂ and nitrate reduction. These relations suggest that lithologic sources of DOC and sulfides are important sources of electrons at these sites but surface‐derived sources of DOC are not. A review of published rates suggests that denitrification tends to occur more quickly when linked with sulfide oxidation than with carbon oxidation.