The invention provides a process for preparing polyether carbonate polyols by addition of alkylene oxides and carbon dioxide onto H-functional starter substance in the presence of a double metal cyanide (DMC) catalyst or in the presence of a metal complex catalyst based on the metals zinc and/or cobalt, wherein (γ) alkylene oxide and carbon dioxide are added onto H-functional starter substance in a reactor in the presence of a double metal cyanide catalyst or in the presence of a metal complex catalyst based on the metals zinc and/or cobalt, wherein a reaction mixture comprising the polyether carbonate polyol is obtained, and wherein (δ) the reaction mixture obtained in step (γ) remains in the reactor or is transferred continuously into a postreactor, wherein the content of free alkylene oxide in the reaction mixture is reduced in each case in the manner of a postreaction, characterized in that a component K is added during the postreaction, component K being selected from at least one compound containing a phosphorus-oxygen-hydrogen group.