Long‐range ordering in a (GaP)2/(InP)2short‐period superlattice and a Ga0.525In0.475P buffer layer grown on a (001)GaAs substrate by gas source molecular beam epitaxy were studied. Transmission electron microscopy and low‐temperature cathodoluminesence techniques were used to examine the microstructure of the short‐period superlattice and to determine its band‐gap energy. The superlattice layer was found to have a [001] long‐range ordered structure with a band gap narrowing of about 130 meV, while the Ga0.525In0.475P layer had a 37 meV band‐gap narrowing induced by spontaneous long‐range ordering in the [111] direction. The ordered superlattice layer was found to have a growth‐induced lateral periodic modulation of the composition along the [1¯10] direction. Within the modulating bands, which had a 200 A˚ periodicity, the In composition was found to vary from 42 to 56% while the Ga correspondingly varied between 58 and 44%.