The mechanisms for formation of interlayers at the interface of GaAs on (In, Ga)P and In0.15Ga0.85As0.7P0.3 grown by metalorganic vapor phase epitaxy have been studied by capacitance voltage profiling of the electron concentration. Shallow In-rich quantum wells (QWs) are formed in (In,Ga)P during growth interruptions, under PH3 stabilization, These QWs are seen in C-V profiles, but not in photoluminescence and X-ray diffraction. Stabilization of (In,Ga)P under AsH3 yields thin (1-2nm thick) (In,Ga)(As,P) interlayers. which are observed by X-ray diffraction and capacitance-voltage profiles, Under optimized conditions. these interlayers exhibit negligibly small band offsets. When growing GaAs on quaternary (In,Ga)(As,P). interlayers can be avoided even at high growth temperatures and long growth interruptions. Independent on the chosen growth conditions, the (In,Ga)P and (In,Ga)(As,P) layers as well as the investigated interfaces are practically free of defect levels as determined by deep-level transient Fourier spectroscopy. (C) 2002 Elsevier Science B.V. All rights reserved.