Plastic deformation produced by phosphorus diffusion into a (001) silicon surface has been studied by transmission electron microscopy. The lattice parameter differences in regions of steep solute concentration gradient are accommodated by a crossed grid of edge dislocations having Burgers vectora/2[110] anda/2[1¯10]. The long edge dislocations end at nodes, which suggests that they are formed by dislocation reactions between pairs of dislocations that can glide into the crystal on {111} planes.