Contamination of soil by polycyclic aromatic hydrocarbons is frequently associated with non-aqueous-phase liquids. Measurement of the catabolic potential of a soil or determination of the biodegradable fraction of a contaminant can be done using a slurried soil respirometric system, This work assessed the impact of increasing the concentration of transformer oil and soil:water ratio on the microbial catabolism of [C-14]phenanthrene to (CO2)-C-14 by a phenanthrene-degrading inoculum. Slurrying (1:1, 1:2, 1:3 and 1:5 soil:water ratios) consistently resulted in statistically higher rates and extents of mineralisation than the non-slurried system (2:1 soil:water ratio: P<0.01). The maximum extents of mineralisation observed occurred in the 1:2-1:5 soil:water ratio microcosms irrespective of transformer oil concentration. Transformer oil concentrations investigated displayed no statistically significant effect on total mineralisation (P > 0,05). Soil slurries 1 :2 or greater, but less than 1 :5 (soil: water), are recommended for bioassay determinations of total contaminant bioavailability due to greater overall mineralisation and improved reproducibility. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.}