It is compelling to eliminate inactive phosphorus in industrial POCl3 diffused emitters (also referred as electron collectors) for high efficiency silicon solar cells. Lately significant emitter quality improvement was demonstrated via flowing oxygen during drive-in. However, the underlying physical mechanisms for the improvements are still unclear. The traditional explanation involving an oxide barrier layer do not appear consistent with all the recently measured data. We propose a new hypothesis, “free phosphorus re-oxidation” to explain more completely the role of oxygen in the POCl3 diffusion process and in particular the means by which oxygen-rich diffusion environments can reduce the flux of phosphorous into the silicon. Characterization techniques including secondary ion mass spectrometry and X-ray photoelectron spectroscopy were carried out on eight distinct diffusion scenarios to study the chemical compositions of the phosphosilicate glass, silicon oxide (SiO2) and silicon regions. We demonstrate that even in samples with similar SiO2 regions, fewer free phosphorus atoms were available for diffusion in an oxygen-rich ambient. A qualitative model is presented to explain this in terms of the re-oxidation of free phosphorous atoms.