Alkane solubilities were measured experimentally for high-density biomass. The resulting Henry＇s law constants for propane were found to decrease significantly for both dense yeast suspensions and an actual propane-degrading biofilm consortium. At the biomass densities of a typical biofilm, propane solubility was about an order of magnitude greater than that in pure water. For example, a dense biofilm had a propane Henry＇s law constant of 0.09 +/- 0.04 atm m(3) mol(-1) compared to 0.6 +/- 0.1 atm m(3) mol(-1) measured in pure water. The results were modeled with mixing rules and compared with octanol-water mixtures. Hydrogels (agar) and salts decreased the alkane solubility. By considering a theoretical solubility of propane in dry biomass, estimates were made of intrinsic Henry＇s law constants for propane in pure yeast and biomass, which were 13 +/- 2 and 5 +/- 2 atm kg biomass mol-l for yeast and biofilm consortium, respectively. (C) 2000 John Wiley & Sons, inc.