Volatile hydrocarbon biodegradation by a mixed-bacterial culture during growth on Bow River crude oil was investigated using solid phase microextraction (SPME). Inoculum treatments were examined in relation to C(5)-C(11) hydrocarbon degradation. Up to 1600 mg/l biomass (dry weight) was tested without achieving significant volatile hydrocarbon partitioning and affecting analysis. Inoculum age rather than concentration had the most profound impact on biodegradation. When late log phase crude oil-grown inocula were used, C(5)-C(11) biodegradation reached 55-60%; methylcyclohexane and other branched compounds eluting before n-C(8) were recalcitrant. Increasing the late log inoculum concentration from 0.63 to 63 mg/l resulted in a twofold increase in degradation rate without improving the substrate range. Methylcyclohexane recalcitrance was correlated with reduced levels of hydrocarbon-degrading bacteria and volatile hydrocarbon evaporation from the inoculum flasks. A decreased lag phase prior to degradation was observed when using early stationary phase cultures as inocula and most compounds up to C(11), including methylcyclohexane, were biodegraded.