The reaction front during leaching of cobalt-aluminum alloys to form skeletal cobalt is sharp and well-defined. In situ milling of a sample by dual-beam electron microscope/focused ion beam provides a high-resolution image of the internal catalytic structure, which is similar to that found for skeletal copper but on a finer scale. The leaching kinetics fit the Avrami-Erofe'ev nucleation-and-growth model with an exponent equal to 1. This represents a rate-limiting step involving lineal (one-dimensional) growth of numerous pore nuclei randomly distributed across the reaction interface, but not such that it is covered completely to fit the shrinking unreacted-core (envelope) kinetic model. The coalescing of product cobalt metal around the activated nuclei focuses the pore growth to being mostly lineal. The reaction rate was inversely proportional to the original mean particle size and has an activation energy of 72 +/- 4 kJ/mol. A quantitative model is determined.