Mass balance and transfer of oxygen has been studied from oxygen microbubbles to low dissolved oxygen ground water passing through variously configured pilot scale vertical slice test cell (VSTC) under aseptic conditions (sodium azide treated). With O2 micorbubble injection at time 'O' under static conditions, initial retentions could be determined. Subsequently, with ground water flow initiated, dissolved oxygen measurements were made by drawing samples from the back face of the cell as well as input and outflow groundwater. From 5.4 to 59% of the oxygen microbubbles injected was transferred to the groundwater. With approximately 12% committed to biodegrade the surfactant used for oxygen microbubble formation, the difference would be available for in-situ biodegradation. The overall Kla's ranged from 0.01 to .09 hr-1 but intermittent O2 microbubble injection should keep the Kla's high. Maximum ΔD.O. ranged from 3.3 to 8.8. The role of oxygen microbubbles for in-situ aerobic biodegradation looks encouraging, and retention and performance during prolonged testing have been markedly improved.