Addition of toluene into slurry phase laboratory microcosm is proposed in order to increase desorption rate of hydrocarbons and as an alternative to improve bioavailability of hydrocarbon in aged soils. Our studies showed that toluene has a positive effect on desorption of total petroleum hydrocarbons (TPH). Addition of 14 000 mg toluene/kg of soil, in highly polluted soil, increased the consumption rate of hydrocarbons three times in comparison to control without solvent. In 30 days the initial TPH concentration in soil, 292 000 mg/kg, diminished 45%. Although toluene was able to dissolve complex organic compounds such as asphaltene fraction, it probably yielded a highly toxic toluene-hydrocarbons phase. The inhibitory effect of toluene-TPH was also studied. A substrate inhibition model was used: the k(m) and k(i) constants were 57 and 490 mg TPH/L liquid phase, respectively. Experimental data were well described when the proposed model included sequential desorption and biodegradation phenomena. Damkohler number evaluation showed that rate of mass transfer was the limiting step in overall biodegradation in nonsolvent control. When high concentration of toluene was added, then bioreaction was the limiting step, but inhibitory effect should be considered. However, toluene addition at low concentrations facilitates the biodegradation of aromatic compounds.