At pH 7.0, in air, 20% of the total electron flux through xanthine oxidase can be accounted for in terms of the univalent reduction of oxygen. The fraction of the total flux of electrons which traversed the univalent pathway to oxygen was increased by raising the pH and by raising the oxygen tension. It was further shown that at any given pH and oxygen tension, the amount of univalently reduced oxygen, which was detectable in terms of the reduction of cytochrome c, rose as the turnover rate of the enzyme was decreased by decreasing the concentration of xanthine. This effect of xanthine was more pronounced at pH 7.0 than at pH 10.0. Another reflection of this same phenomenon was a difference in K_m for xanthine measured in terms of urate production as compared to K_m for xanthine measured in terms of cytochrome c reduction. Here too the differences were diminished as the pH and the oxygen tension were raised. The quantitative aspects of these phenomena are presented as well as an explanation which is consistent with all of the observations and which was, in fact, predictive of several of them.