A process for preparing polyether polyols by reaction of the following starting materials: a) one or more alkylene oxides and, if appropriate, carbon dioxide and also b) one or more H-functional starter substances, in the presence of a catalyst, in a reaction unit having a plurality of parallel layers A, B which are microstructured so that each layer has a multiplicity of channels which are arranged parallel to one another and form a continuous flow path from one side of the plate to the opposite side of this, wherein a distribution device for introduction of the starting materials and the catalyst is provided at one end of the channels of the layers A and a collection device for the reaction mixture is provided at the other end of these is proposed.