The sorting of plastic types on the basis of differing density has important industrial application in the recycling industry. Some important packaging plastics, such as PVC and PET have similar densities, making density separation either unsuitable or requiring very careful control. The behaviour of PVC and PET in a LARCODEMS dense medium separator has been investigated using calcium chloride solutions as the medium. It has been shown that particle thickness and surface conditioning can have a significant influence on the plastic＇s behaviour within the separator. The results can be explained by considering the forces acting on the particles, with surface tension and buoyancy being of particular importance. The possibility of controlling the wettability of the plastic to improve the separation of PET and PVC is discussed.