A method for processing an olefin-containing product stream that contains dimethyl ether as a by-product is described. Such a product stream arises, for example, in the synthesis of olefin from methanol. For separation of the dimethyl ether from the product stream, it is proposed that at least a partial stream chiefly containing C.sub.3 hydrocarbons is separated from the product stream by fractionation, and is sent to a rectification column (C.sub.3 splitter) (16) for separation of propylene and propane. The dimethyl ether goes together with the propane into the bottom of the rectification column in the rectification process and can be withdrawn (42). A substantially pure propylene product (41), which contains at most only traces of dimethyl ether, can be removed from the top of the rectification column.