Abstract: In an upward gas stream sifter for separating sheet-like pieces of paper and plastic in a wetted mixture, the gas is heated to at least 60.degree. C. in order to dry surface moisture on the plastic. The temperature of the gas is not so high that the plastic shrivels. This increases the difference in density between the pieces of plastic and wet paper, thus assisting the separation.Optionally some gas may enter the mixture feed duct through a gas permeable part of its wall, to prevent accumulation of mixture on the wall.If the mixture is derived from household rubbish and the plastic is mostly polyethylene the gas may enter the sifter at 120.degree. C. and leave it at about 70.degree. C.