The water sorbed by oligo(ethylene oxide) monododecyl ether of the homologous series from one up to eight oxyethylene groups and the repeat distance in the case of the lamellar phase at constant relative humidities (RH = 84.2 and 97.0%) were determined by the isopiestic method and X-ray diffraction, respectively, at 25 degrees C. Further, deuterium NMR relaxation of deuterium oxide in large concentration range of surfactant dispersions in heavy water was measured at the same temperature. In the case of the lamellar phase the surface area of surfactant and the thickness of the hydrophobic core were estimated from the repeat distance using the composition of the samples and the molecular volumes. For all surfactant dispersions the effective water binding energy was deduced from the deuterium relaxation times. It was found that the dependence of the water sorbed and of the surface area at constant relative humidity as well as of the effective water binding energy on the number of the oxyethylene groups can be described by linear functions. These findings are discussed with respect to the parameters which exhibit gradual changes along the ethylene oxide chain and in the light of a dynamic picture. The water sorbed increases by 0.7 and 2.1 molecules of water at the relative humidities of 84.2 and 97.0% (25 degrees C), respectively, the surface area of the surfactant by 3.7 Angstrom(2) at the relative humidity of 97.0% (25 degrees C), and the effective water binding energy by 7 kJ/mol by adding one oxyethylene group.