The present invention refers to a method for the preparation of a polymer polyol which comprises: (i) preparing an intermediate in a first reactor by polymerizing a mixture comprising: (a) a base polyol in an amount from 60 to 100 wt % of the total amount of base polyol, (b) at least one ethylenically unsaturated monomer, (c) an acylperoxide radical initiator in an amount from 50 to 90 wt % of the total amount of acylperoxide radical initiator, and (d) a preformed stabilizer in an amount from 70 to 100 wt % of the total amount of preformed stabilizer or a macromer in an amount of from 70 to 100 wt % of the total amount of macromer; (ii) polymerizing in a second reactor a mixture comprising the intermediate prepared in step (i), the balance acylperoxide radical initiator proportion, the balance preformed stabilizer or macromer proportion and the balance base polyol proportion; wherein the at least one ethylenically unsaturated monomer is added only to the first reactor.