The quenched microstructures of self-propagating reactions in Al/Ni and Al/Monel foils are examined. The quenched reaction fronts in both films are shown to be approximately 30μm long and exhibit a continuous evolution from an as-deposited to a fully reacted microstructure. In both multilayers the first phase to form is isostructural with Al9Co2, and nucleates on the Al side of the layer interface. Growth of the intermetallic occurs in a two step process. First the grains grow laterally along the interface until they impinge on each other, followed by growth normal to the interface into the Al layer. It is shown that planarity of this interface during the latter growth stage is determined by the width of the intermetallic grains immediately prior to their growth normal to the interface into the Al layer. On the basis of these observations it is argued that in modeling these reactions diffusion through the intermetallic during the reaction must be considered, and that atomic diffusion may be considerably more three- dimensional in practice than the current models suggest.