Silicide formation, growth kinetics, and phase sequences have been investigated with lateral-diffusion, thin-film couples formed by overlapping deposited layers of Ni and Si on an inert substrate. The lateral extent (3 to 100 μm) of the silicide region is determined by electron microscope and microprobe measurements. For annealing temperatures between 400 and 800 °C, the growth of the silicides follows a (time) 1/2 dependence at a growth rate similar to that in conventional (planar) thin-film couples. In Ni-Si lateral diffusion couples, Ni2Si is the first phase formed; it grows to a length of about 25 μm (at 600 °C) and then multiple phases appear with the same sequence as that found in bulk diffusion couples. Lateral-diffusion couples thus provide the transition between thin-film and bulk behavior.