The effects of Si doping on ordering and domain structures in GaInP grown by organometallic vapour phase epitaxy on (001) GaAs singular and vicinal substrates at 620 degrees C have been investigated by transmission electron microscopy (TEM) and transmission electron diffraction (TED). TEM results show that the behaviour of antiphase boundaries (APBs) in the singular samples differs from that of the vicinal samples. As the carrier concentration increases, the density of APBs in the vicinal samples increases slightly, whilst that of the singular samples varies insignificantly. APBs are inclined some degrees from the [001] growth surface. TEM results show that for the highly doped samples, the ordered domains contain dark mottled contrast corresponding to either much less ordered regions or disordered regions. TED results show that the ordering is present in layers exceeding a concentration of similar to 2.0 X 10(18) cm(-3).