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Fully stable isotope-labeled toluene was used as a model volatile petroleum hydrocarbon (VPH) to investigate the mineralization of VPHs and soil organic matter (SOM) by soil microorganisms in the presence and absence of 2%%(w/w) wood-chip biochar or bitumen-derived activated carbon. Biodegradation of 13C7 -toluene was monitored in batch microcosms for 19 days by measuring headspace CO2 and toluene concentrations. A numerical carbon mass balance model was developed to interpret the experimental observations. Increased 44CO2 and 45CO2 headspace concentrations following 13C7 -toluene addition as compared to live and abiotic controls indicated a stimulation of microbial activity. By fitting a maximum specific growth rate (μmax) of 0.6 h −1 (in line with published rates), it was possible to match model predictions of 45CO2 and 44CO2 concentrations with the experimentally determined data. Half-saturation constants (Ks) of 4.06 × 103 mgL −1, 7.76 × 102 mgL −1 and 1.83 × 102 mgL −1 were predicted for soil, soil & BC and soil & AC respectively, much higher than previously reported values. Differences in the half-saturation constant suggests that sorbent amendment affects the microbial ecology, by making microorganisms which can utilize substrates at lower concentrations more competitive. Consequently, amending soils with 2% BC or AC changed the biokinetics of VPH degradation by rendering VPHs less bioavailable, but did not appear to have any detrimental effects on VPH degradation and may serve as a sustainable, cost-effective approach for enhancing the natural attenuation of VPHs in soil on the long term.