Trace element concentrations and isotopic ratios are frequently used to study the behavior of high-temperature fluids in both metamorphic and igneous systems. Many theoretical formulations of the effects of fluid migration on trace elements have assumed instantaneous reequilibration between the migrating fluid and the solid material through which it is passing. This paper investigates the additional effects which arise when equilibration is not instantaneous due to a limited rate of diffusion in the solid, using an analytical steady state solution to a set of partial differential equations describing the exchange of trace elements between the fluid and the solid during the migration of the fluid. It is found that the rate of transport of the trace element variations depends on the spatial and temporal scale of the initial variation in the trace element concentration in the fluid as well as porosity, distribution coefficient, diffusion coefficient in the solid, and channel size. In addition, the amplitude of any initial trace element variations decreases with increasing distance of fluid migration at a rate which depends on the scale of the initial trace element variation, the parameters listed above, and the velocity of fluid migration. The effect of varying the system parameters controlling transport rate and damping is investigated for the case of basaltic magma migrating through a lherzolite matrix. The operation of diffusion-limited reequilibration during fluid migration can be recognized in nature from studies of the altered regions surrounding veins in peridotite and from time stratigraphic studies of erupted lavas. Consideration of one geochemical study for the East Pacific Rise generates reasonable estimates of migration velocity and suggests that the magma has migrated primarily through macroscopic channels rather than along grain boundaries. ¿ American Geophysical Union 1993