Ga0.5ln0.5P layers grown by organometallic vapor phase epitaxy at rates of between 0.1 and 4.0 &mgr;m/h on exactly (001)‐oriented GaAs substrates have been studied using atomic force microscopy. The surface is found to be covered by islands several monolayers in height that are elongated in the [110] direction. The edges of the islands are formed of clearly resolved bilayer (5.9 A˚) steps. Monolayer steps are rare and no steps larger than 6 A˚ were observed. These observations explain the nature of the order twin boundaries in ordered GaInP grown on exactly (001)‐oriented substrates. The (001) domain laminae are always found to consist of an even number of monolayers. For bilayer steps, the domain thickness will be twice the number of steps moving across the surface before the direction of step motion switches due to the undulating nature of the surface. This switch in the direction of step motion at a particular location on the surface produces the order twin boundaries observed. ©1995 American Institute of Physics.