High-saturation pools of dense nonaqueous phase liquid (DNAPL) are long-term sources of groundwater contamination at many hazardous-waste sites. DNAPL pools consist of a high saturation zone with slow dissolution overlaid by a transition zone with lower saturations and more rapid dissolution. Effects of biological activity on pool dissolution must be understood to evaluate and implement bioremediation strategies. Bioenhanced dissolution of tetrachloroethene (PCE) in transition zones of high-saturation pools was investigated in a custom-designed 5-cm flow cell. Experiments were conducted to characterize mass transfer following DNAPL emplacement, with and without an active microbial culture capable of reductive dehalogenation. For average pool saturations < or = 0.55, mass transfer during biodegradation was enhanced by factors of 4-13, due primarily to high mass flux of PCE degradation products. However, at an average pool saturation of 0.74, mass transfer was enhanced by factors less than 1.5. Mass transfer was significantly greater from pools with an observable transition zone than without. Advective flow through multiphase transition zones enhanced dissolution and biological activity. These laboratory-scale experimental results suggest that biotechnologies may be effective remediation strategies for depletion of source zones within pool transition zones.