Interfaces (IFs) of In0.485Ga0.515P/GaAs/In0.485Ga0.515P quantum wells (QWs) grown by LP-MOVPE technology were studied, and a significant influence of growth conditions on properties of the QWs was observed and theoretically analysed. The paper shows that a low-band-gap ternary or quaternary interlayer (IL) between the bottom InGaP barrier and the GaAs well layer is the general reason for the deterioration of the IF quality. Such an IL forms spontaneously during the growth. Its formation can be enhanced using an inappropriate growth switching sequence and temperature. An As-to-P exchange process was found to be the dominant mechanism of the formation of the IL at a growth temperature of 560 degrees C. An In0.485Ga0.515As monolayer (ML), created by the P-to-As exchange mechanism, is always present at the InGaP/GaAs IF. Even such an ML can considerably deteriorate properties of the QW. A proper switching sequence with thin GaP ILs grown at the Ifs can eliminate the influence of spontaneous intermixing processes. We have calculated the quantized energy levels of QWs, which contained IL of various compositions and thicknesses. The results of calculation were compared with experimentally determined energies obtained from photoluminescence (PL) measurements. Based on this comparison, thicknesses of the ILs were estimated. Only small influence of As interdiffusion into the barriers and a negligible influence of indium carry-over into the GaAs well material were observed. From an analysis of the results it follows that about 1 ML thick ILs are spontaneously formed at the growth conditions used.