Narrow InGaP/GaAs/InGaP and InGaP/InGaAs/InGaP quantum wells (QWs) prepared by MOVPE were studied by photoluminescence and HRXRD in this work. A detailed analysis of the results has shown the presence of band gap fluctuations at the interfaces in the InGaP/(In)GaAs/InGaP system. A series of experiments with various thickness of QWs from 1 to about 19 monolayers allowed us to distinguish various influences on the interface quality. It is shown that diffusion plays only a negligible role at our growth temperature of 560degreesC. Immiscibility in a thin In1-xGaxAs1-yPy interlayer that forms at the interface along with surface kinetic processes are suggested to be the driving force for composition variations at the interfaces. The impact of MOVPE growth techniques minimising the inhomogeneities at the interface is discussed. (C) 2002 Published by Elsevier Science B.V.