The surface tension of poly(ethylene oxide)(PEO) solution has been measured as a function of molecular weight. The PEO molecule is known to be surface active and can form a monolayer at an air/water interface as well as a self-assembled film from an aqueous solution. Conventional adsorption behaviour is observed at a low polymer concentration, but additional surface tension reduction behaviour is found at a high polymer concentration (M. W. Kim and B. H. Cao, Europhys. Lett., 1993, 24, 229). The behaviour of a self-assembled adsorbed film at low PEO concentration is found to be very similar to that of a PEO monolayer at an air/water interface. The surface tension reduction due to this self-assembled film at low concentration is independent of the molecular weight. However, the additional surface tension reduction in the high polymer concentration regime depends strongly on the molecular weight. This molecular weight dependence is likely due to the hydrophilic and lipophilic balance (HLB) of PEO.