The influence of the molecular weight of polyethylene oxide (PEO) on the results from several methods for determining its concentration in aqueous solutions has been investigated. Modified versions of the complexation reactions of PEO with molybdophosphoric acid and with tetraiodobismuthate give results that are independent of molecular-weight effects for the range ca. 400-10(6). The reaction with KBiI(4) is the less accurate. Oxidative digestion is also independent of molecular-weight, but impurities easily obscure the measurements. Other methods studied included interferometry, viscometry, and complexation with tannic acid. Interferometry and the reaction with tannic acid were only independent of molecular-weight for M > 4000. With viscometry very poor results were obtained.