The fates and the rates of metabolism of acetate, trimethylamine, methylamine, and methanol were examined to determine the significance of these compounds as in situ methane precursors in surface sediments of an intertidal zone in Maine. Concentrations of these potential methane precursors were generally <3 muM, with the exception of sediments containing fragments of the seaweed Ascophyllum nodosum, in which acetate was 96 muM. [2-C]acetate turnover in all samples was rapid (turnover time <2 h), with CO(2) as the primary product. [C]trimethylamine and methylamine turnover times were slower (>8 h) and were characterized by formation of both CH(4) and CO(2). Ratios of CH(4)/CO(2) from [C]trimethylamine and methylamine in uninhibited sediments indicated that a significant fraction of these substrates were catabolized via a non-methanogenic process. Data from inhibition experiments involving sodium molybdate and 2-bromoethanesulfonic acid supported this interpretation. [C]methanol was oxidized relatively slowly compared with the other substrates and was catabolized mainly to CO(2). Results from experiments with molybdate and 2-bromoethanesulfonic acid suggested that methanol was oxidized primarily through sulfate reduction. In Lowes Cove sediments, trimethylamine accounted for 35.1 to 61.1% of total methane production.