We studied single quantum wells (SQWs) of an InGaP/GaAs material system grown by gas-source molecular beam epitaxy (GSMBE) to develop an optimized source-supply switching sequence that will enable abrupt flat InGaP/GaAs heterointerfaces. By investing the photoluminescence (PL) peak energy and the full-width at half-maximum (FWHM) of the SQWs, we clarified the mechanism that degrades the abruptness at the heterointerface, and found that it differs between the InGaP-to-GaAs and GaAs-to-InGaP interfaces. We believe that the dominant factors for the respective interfaces were the As/P exchange reaction during the As-2 exposure and the existence of excess As atoms after the interruption of As-2 exposure; both factors resulted in transit layers at the heterointerfaces.