The isotopic composition of dissolved O2 in seawater, expressed as the Δ18O of O2, is unique among the bioactive tracers of the aphotic zone in that it is not linearly related to oxygen utilization via the stoichiometry of organic matter decomposition. In fact, Δ18O of O2 depends on the history of water mixing and O2 consumption in the sample studied (Craig and Kroopnick, 1970; Kroopnick and Craig, 1976). For this reason, the variation of Δ18O of O2 with O2 concentration depends on regional circulation patterns and oxygen utilization rates. The Δ18O of O2 can be used to characterize these processes by decoupling their effects. As an example of this assertion, we interpret the covariation between the concentration of O2 and its isotopic composition in the Pacific Ocean as reported by Kroopnick (1987), using four simple representations of seawater mixing and respiration. Kroopnick's data are in general accord with an elementary model of isopycnal mixing represented by diffusive exchange and oxygen utilization in the ocean's interior, coupled with atmospheric equilibrium at the point where the isopycnals outcrop at the sea surface. This specific result illustrates the general point that Δ18O of O2 in seawater can serve as an important constraint on more extensive and sophisticated physical models used to estimate rates of oxygen utilization in the deep sea. ¿ American Geophysical Union 1990