Immiscibility in InGaAsP on GaAs substrates grown by metalorganic vapor phase epitaxy (MOVPE) has been investigated. Epitaxial layers of InGaAsP over a wide compositional range were grown on (100) exact and 10 degrees off toward < 111 > A GaAs substrates at the same time under growth temperatures of 630 and 690 degrees C. We examined the immiscibility by using measurements of surface roughness and optical characteristics, and revealed that the calculation based on the strictly regular solution model agreed very well to the immiscibility of InGaAsP grown on 10 degrees off substrates at 690 degrees C. Furthermore, as for the samples grown at 690 degrees C on (100) exact substrates or grown at 630 degrees C, the phase separation was suppressed even in the unstable compositional region, especially in the compositions close to InGaP. This phenomenon is due to influence of the surface reconstruction of InGaAsP layers. (c) 2006 Elsevier B.V. All rights reserved.