The invention relates to a process for removing poly(propylene oxide) from propylene oxide by membrane separation, wherein a membrane having an average pore size of from 0 to 5 nm is used. In said process, a liquid feed comprising propylene oxide and poly(propylene oxide) may be separated by the membrane into a permeate comprising propylene oxide and either no poly(propylene oxide) or poly(propylene oxide) at a concentration which is lower than the poly(propylene oxide) concentration in the feed, and a retentate comprising propylene oxide and poly(propylene oxide) at a concentration which is higher than the poly(propylene oxide) concentration in the feed.