In a random solid solution, if the two atoms have appreciably different sizes, the nearest‐neighbor distances and to a lesser extent the higher neighbor distances will be of three kinds, &ggr;AA, &ggr;AB, and &ggr;BB. The effect produces modulations in the diffuse intensity similar to those produced by short‐range order. The size effect is important when the difference in scattering power is large, the difference in size is large, and the short‐range order is small. The size effect is illustrated by a single crystal pattern of Cu3Au and a powder pattern of Ni3Au2. An asymmetry in the wings about a fundamental reflection is a result of the size effect.