The structure of gallium arsenide and indium phosphide (00 1) surfaces in the metalorganic vapor-phase epitaxy (MOVPE) environment has been investigated. During growth at V/III ratios in excess of 10, both materials are terminated with group V ad-dimers (As or P), alkyl groups and hydrogen atoms. These species sit on top of a complete layer of the group V atoms. As the V/III ratio decreases, the top layer of arsenic or phosphorous desorbs from the surface. However, the resulting structures are different on GaAs and InP (0 0 1). In the former case, the phase transition occurs with gallium out-diffusion and nucleation of elongated islands. These islands have a beta2(2 x 4) structure that contains only 0.75 monolayer of arsenic dimers. The resulting surface is rough, exposing on average six atomic layers. Conversely, on InP (0 0 1), no indium out-diffusion occurs following desorption of the phosphorous ad-dimers. Instead, the underlying P atoms dimerize, forming a (2 x 1) structure with a phosphorous coverage of 1.0 monolayer.