In the presented study we have developed and implemented a methodology for ellipsometry measurements at liquid interfaces that makes it possible to determine the amount adsorbed without assumptions of refractive index or thickness of the adsorbed layer. It was demonstrated that this is possible by combined measurements from different aqueous phases, H2O and D2O, which were shown to have sufficiently different refractive indices. The methodology was tested by studying adsorption of two types of nonionic poly(ethylene glycol) alkyl ether surfactants, CnH2n+1(OC2H4)(m)OH or CnEm at the decane-aqueous interface, where C12E5 was adsorbed from the oil phase and C18E50 from the aqueous phase. The observed plateau values of the adsorbed amounts were 1.38 and 0.93 Mg/M-2 for C12E5 and C18E50, respectively, which is in agreement with the corresponding values of 1.49 and 1.15 Mg/M-2 obtained from applying the Gibbs equation to interfacial tension data for the same systems. We will briefly discuss the adsorption behavior in relation to the molecular structure of the surfactant and the phase behavior of the oil - surfactant- aqueous systems in relation to our experimental results.