In InGaAs/InP heterostructures, interdiffusion of arsenic and phosphorus at the hetero-interfaces during epitaxial growth degrades the interface abruptness. This is a serious problem when a very thin InGaAs/InP quantum well is required. In this paper, spectroscopic and kinetic ellipsometry are used to monitor the As-P exchange in-situ in metal-organic vapor phase epitaxy (MOVPE) with TBAs and TBP as the group V precursors. As a result, it is found that the monitoring of As-P exchange is possible by kinetic ellipsometry and that useful information for improving the gas switching sequence is acquired from such observations. Information from this in-situ kinetic ellipsometry has been confirmed to agree well with ex-situ photoluminescence and transmission electron microscope (TEM) observations.