Plate separators are an effective and often used apparatus for the separation of droplets from liquids, e.g. in the case of oil-water purification . The separation is accomplished by coalescing the droplets on inclined plates . Our previous papers dealt with the separation performance, design and hydrodynamic modelling of plate separators . One important result was that incomplete coalescence of drops, the so-called partial coalescence, often takes place and, under certain circumstances, governs the separation performance . In these cases, plate separators do not work very effectively . This paper offers a report on theoretical and experimental investigations on partial coalescence on inclined plates of plate separators. It reviews the state of the art, gives models for the estimation of the secondary droplet size and deduces a semi-empirical criterion to predict the conditions for its occurrence . The conclusion is that the occurrence depends, firstly, on the liquids involved and, secondly, on the primary drop size . Additionally, the performance of partial coalescence is mainly subject to interfacial effects whereas hydrodynamics play only a minor role .