Low viscosity polymer polyols having high styrene/acrylonitrile ratios and good stability may be achieved by the use of epoxy resin-modified polyols as the base polyols. The styrene/acrylo-nitrile ratio may be as high as 95/5 and even 100/0 in the polymer polyols. The epoxy resin-modified polyols useful as base polyols may be made by reacting a polyol initiator having an active hydrogen functionality of 3 to 8 and one or more alkylene oxides with an epoxy resin. It is preferred that only a small part of the epoxy resin-modified base polyol be initially charged to the reactor, and that the balance, a relatively larger part, be a part of the monomer feed stream to the reactor.