Groundwater contamination with organic compounds, especially leaked or spilled fuels, is a serious and growing environmental problem which is difficult to remediate. Alcohol additives in gasoline present more urgent problems because of their relatively high mobility in the subsurface. This study focused on the subsurface biodegradation of two octane-boosting additives, methanol and tertiary butyl alcohol (TBA). A microcosm study was undertaken to determine the <u>in situ</u> biotransformation rates. While both alcohols were found to be amenable to biodegradation in the subsurface systems, the methanol removal rate was much greater than the TBA rate. By using sets of microcosms, several chemical additives were used in an effort to determine their effects on biodegradation. Organic substrates were added to microcosms containing low concentrations of TBA (about 1 mg/L) without evidence of stimulation or inhibition. Variation of pH (from pH 5.1 to 8.8) in microcosms containing methanol and TBA was part of the next experiment. This was combined with the addition of alternative electron acceptors, nitrate and sulfate. Variation of pH in non-amended and sulfate-dosed (100 mg/L) microcosms did not enhance the biodegradation of either alcohol. Nitrate addition inhibited both methanol and TBA biodegradation at lower pH levels, but enhanced methanol degradation when combined with base. TBA degradation was not affected by base addition except in the highest (initial pH 8.8) base dose. These microcosms exhibited TBA removal rates which were uniformly higher than all other treatments. / M.S.