One of the mechanisms of transfer of material through water-saturated rocks is solute diffusion from places of high concentration to places of lower concentration. The distance of movement through rocks of given concentration fronts has been studied experimentally as a function of time, temperature, and concentration of the reservoir. The accord of the experimental work with general diffusion theory has made it possible to present a nondimensional equation for the movement of diffusing ions through rocks. This equation makes it possible to calculate the distance of movement of selected concentration fronts over a wide range of solution composition. The rate of advance of a given concentration front is independent of the permeability or porosity of the limestones studied, but the amount of material transferred depends upon the porosity in the direction of diffusion. Methods of measuring this porosity are discussed, and values determined for several limestones. Solution of several sample problems indicates that solute diffusion through intergranular spaces in rocks is a more effective geologic process, even at low temperatures, than is generally believed.