Solutions of one-dimensional transport equations indicate that the time-space evolution of oxygen isotope exchange between rock and infiltrating fluid is dependent on 1) the rate of fluid infiltration, 2) the dispersive properties of the rock matrix, 3) the rate of isotopic exchange, and 4) the mass oxygen ratio. The geometry of isotopic exchange fronts developed in a rock sequence depends on the interplay between these first three parameters. The change in isotopic ratio in a rock depends on its position in the flowpath and the rate of isotopic exchange with the fluid in addition to the cumulative fluid flux. Thus conventional water/rock (W/R) ratios will also depend on position in flow paths. Absence of significant 18O depletions in rock sequences does not require low W/R ratios, but may only mean that the rock is in a rock-dominated segment of a flowpath or alternatively that fluid infiltration was characterized by nonequilibrium exchange. ¿American Geophysical Union 1991