AbstractThe molecular weight averages of narrow and broad distribution commercial polyethylene oxide have been estimated by controlled precipitation from iso‐octane/benzene solutions by cooling from 333 to 273 K. Estimates of the distributions, which compared favourably with those determined by gel permeation chromatography (gpc), were obtained from a calibration procedure between the temperature of initial onset of precipitation and the viscosity average molecular weight.The chief advantages of this procedure were that it enabled molecular weight characteristics to be determined rapidly and with small amounts of materials when g.p.c. was not readily available. The method is used to assess various fractionation procedure