Abstract: The impact of the addition of a polymer (polyethylene glycol) with a molecular weight of 20 kDa on the structure of micellar systems of an anionic surfactant (sodium oleate) in aqueous solutions is investigated via small-angle neutron scattering. The structure and interaction parameters of micelles (micelle aggregation number, degree of ionization, axial ratio, average diameter, charge, inverse screening length, and surface potential) are compared in a solution with and without the addition of the polymer. Using the concentration dependences of the experimental data on surface tension, parameters such as the critical micelle concentration, area per molecule, surface activity, surface excess, and critical aggregation concentration are determined in the case of complex solutions. The observed effect of the addition of the polymer on the behavior of a micellar solution of sodium oleate can be related to a change in the surface activity of the surfactant in the presence of the polymer. As a result of comparison with previous experimental data, it is concluded that the weight of the polymer significantly affects the structural properties of sodium oleate–polyethylene glycol mixed solutions.