The potential of lead isotopes as tracers of methyl tert-butyl ether (MTBE) in groundwater was assessed in a study that included: an evaluation of organolead’s chemical affinity with individual gasoline hydrocarbon constituents; gasoline/water partitioning experiments; and site-specific investigations of MTBE- impacted groundwater. Concentrations of organolead were correlated with concentrations of MTBE, pentane, benzene, toluene, xylenes, ethylbenzene, 1,2,4-trimethylbenzene, and methyl- cyclohexane. Significant correlations (R² > 0.98) were observed between organolead and methylcyclohexane, MTBE, and benzene; the latter two being the most water-soluble gasoline constituents. To evaluate the possible role of MTBE in the transport of organolead from a product into the aqueous phase, two unleaded gasoline–aqueous partitioning experiments were performed. Each unleaded gasoline was sequentially extracted with fresh volumes of water over a period of ~3 weeks; a strong systematic relationship (R² = 0.998) was observed between lead and MTBE in the aqueous phase, indicating that the organo- lead–MTBE chemical affinity in unleaded gasoline was maintained as these gasoline constituents partitioned into water. The lead isotopic equilibrium between unleaded gasoline and aqueous- phase lead also supports this conclusion. Site-specific analyses of groundwater impacted by MTBE corroborate the laboratory results, showing that groundwater impacted by MTBE releases exhibit lead isotopic ratios identical to those of unleaded gasoline responsible for the release. Our results also suggest that the fate/transport of MTBE plumes can be monitored by integrated lead/MTBE data, possibly to distances exceeding 500 m from the source of the release.