The microstructures of In0.5Ga0.5P alloys grown on (100) GaAs by MOVPE have been characterized with cross-section TEM and their optical emission examined with photoluminescence at low temperatures. All the alloys exhibit spinodal-like decomposition with compositional modulations along directions in the growth plane. Alloys grown at 775 °C have the highest emission energy, 2.0 eV; growth at 675°C gave the lowest, 1.89 eV, due to strong CuPt-type ordering of In and Ga. The ordered domains are platelets 20 to 200 nm wide and 10-20 nm thick, with antiphase boundaries 1-2 nm apart. We have also formed "unicompositional" quantum wells of thin (1.3-20 nm) ordered layers grown at 675°C between disordered barriers grown at 750°C. Ordering is found only in the active layer, with domains similar to those of thick layers. The emission energy increases by 90 meV as the well thickness is decreased from 10 to 1.3 nm, thus demonstrating quantum size effects solely through disorder-order phenomena.