Considerable attention has been paid to the development of mathematical models to describe fluid-solid reaction systems as they play a significant role in many chemical, metallurgical, and other engineering areas. The present work is aimed at examining one of the assumptions generally made in the mathematical modeling of fluid-solid reaction systems; namely, that the bulk concentration of the fluid reactant is uniform around the solid surface. This is a reasonable assumption under well-mixed conditions of the fluid phase. However, under certain conditions, concentration gradients in the axial direction in the fluid near the surface may be present; a packed bed is an example of such a case. It is of interest to investigate how large the concentration gradient should be before the assumption of a uniform bulk concentration around a pellet to cause a significant error. The effect of a fluid concentration gradient has been studied for a catalytic reaction on the external surface of a nonporous sphere. Petersen et al. (1964) found that external concentration gradients do not significantly affect the overall reaction rate except for the case of a second-order reaction when the reactant concentration drops from its maximum value to zero over the distance of a particle diameter, which is a rather unlikely situation in practice. Similar results were obtained by Acrivos and Chambre (1957), who recommended the use of this approximation with caution when a series of consecutive reactions takes place. In many fluid-solid reactions, the reaction progresses towards the interior of the pellet as the solid reactant near the external surface is consumed, leaving behind either a porous solid product or inert solid matrix. Although it is generally believed that a bulk concentration gradient does not significantly affect the behavior of most noncatalytic fluid-solid reactions of industrial relevance, no formal verification of this statement has yet been provided in the literature. In this work, the validity of this hypothesis is tested for a simple fluid-solid reaction configuration.