Density measurements have been performed on water solutions of nonionic surfactants oligooxyethylene glycol-monoether (C12Ej with j = 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure surfactants in their Liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethylene (EO) bulk liquid phases. The deviation from ideality reduces as j increases and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T-cross), at which the C12Ej aqueous solution density coincides with the solvent one up to 50 wt % has been found in all the E-j species. The T-cross of the different E-j results to be scaling temperatures for the excess density of these surfactants. The sphere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the Ej species. In the case of j = 6, 7, and 8 the values of the sphere-to-rod transition temperatures are known and these differences are all about 20 degreesC. For C12E5 only rods were experimentally observed and, coherently, the previous consideration led to a sphere to-rod transition temperature of -7 degreesC. The difference between the critical and the crossing temperatures is constant too, at about 15 degreesC for j < 8, whereas for j = 8 it is 7 C. In this case the densities of the separated phases, two degrees above the critical temperature, result to be the same, within 10(-6) g/cm(3). It implies that, when slightly above the critical temperature, C12E8-water solution undergoes phase separate as it was in microgravitational condition. In our analysis, the C12Ej solution is considered a three-component system: the oil core, the bulk water, and the region containing the aggregate interface, a mixture of water molecules, and EO units. Fixing the density of the oil core and of the bulk water phase respectively equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that-the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusion follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur.