Past practice of co-disposing priority pollutants with municipal solid waste (MSW) has led to the placement of more than 150 MSW landfills in the US on the National Priorities List of Superfund. Interactions between organic contaminants and MSW constituents and the effects of these interactions on contaminant fate are poorly understood. The objective of this study was to evaluate the effects of sorbate-sorbent aging time and sorbent decomposition on toluene bioavailability and fate. The bioavailability of (14)C-toluene sorbed to individual MSW constituents [office paper, newsprint, model food and yard waste, high density polyethylene (HDPE), and poly(vinyl chloride) (PVC)] was evaluated after aging times of 1, 30, and 180d. Biodegradable sorbents were tested in both fresh and anaerobically degraded forms to evaluate the effect of sorbent decomposition. At the termination of bioavailability tests, the distribution of (14)C that was not converted to (14)CO(2) was measured by sequential lipophilic solvent and base extractions of sorbents followed by combustion of extracted sorbents. Increasing the toluene-sorbent aging time reduced the rate of toluene biodegradation for all MSW components except for HDPE. (14)C remaining in sorbents at the completion of bioavailability tests was physically sequestered within and/or covalently bound to sorbent organic matter, and this fraction increased with increasing aging time. Up to 18.6% of (14)C was associated with humic matter (humic and fulvic acids, humin) at the completion of bioavailability tests.