In this study, impediments to hydrocarbon biodegradation in contaminated soils were assessed using chemical and molecular methodologies. Two long-term hydrocarbon contaminated soils were utilised which were similar in physico-chemical properties but differed in the extent of hydrocarbon (C-10-C-40) contamination (S1: 16.5 g kg(-1); S2: 68.9 g kg(-1)). Under enhanced natural attenuation (ENA) conditions, hydrocarbon biodegradation was observed in S1 microcosms (26.4% reduction in C-10-C-40 hydrocarbons), however, ENA was unable to stimulate degradation in S2. Although eubacterial communities (PCR-DGCE analysis) were similar for both soils, the alkB bacterial community was less diverse in S2 presumably due to impacts associated with elevated hydrocarbons. When hydrocarbon bioaccessibility was assessed using HP-beta-CD extraction, large residual concentrations remained in the soil following the extraction procedure. However, when linear regression models were used to predict the endpoints of hydrocarbon degradation, there was no significant difference (P>0.05) between HP-beta-CD predicted and microcosm measured biodegradation endpoints. This data suggested that the lack of hydrocarbon degradation in S2 resulted primarily from limited hydrocarbon bioavailability. (C) 2013 Elsevier B.V. All rights reserved.