Indium-mediated allylation provides remarkable stereo- and regioselectivity, and it proceeds easily and in high yield in aqueous solutions. In spite of its widespread use, there have been few fundamental studies of this reaction. We have developed a photomicrographic technique for measuring rates of reaction of allyl halides at indium surfaces, and we describe the mathematical model for discriminating between diffusion and kinetic control. The measurements demonstrate that this reaction is diffusion controlled, and the minimum value of the heterogeneous rate constant is 1 x 10(-3) cm s(-1). These results broaden the applicability of photomicroscopy for measuring heterogeneous rates of reactions that result in consumption of solid metals.