Expanded polystyrene (EPS) is widely used in works where there are soils with low support capacity, bridges and / or viaducts and road widening. This is due to its properties such as low specific gravity, high strength and low compressibility. In addition, by not releasing pollutants and being totally recyclable, it is susceptible of environmental application. In this way, its use in conjunction with soils is very common. Analyzing the possibilities of rupture, it is observed that the plane where there is a greater probability of failure occurs at the interface between the materials (there is a tendency of sliding of one in relation to the other). In this way, it is essential the analysis of the direct shear parameters, especially the interface friction, to verify the external and internal stabilities by the action of horizontal loads. Thus, this work evaluated the shear strength parameters of EPS in different specific masses (18, 28 and 33.5 kg / m³) with clay and sandy soil interfaces measured at the optimum moisture obtained in the compaction test in normal energy. It was considered as an acceptable error up to 1% difference in the humidity measured in relation to the optimum. In the laboratory, specific mass tests were performed for characterization and direct shear tests at EPS / Solo interfaces according to ASTM D3080 (shear box 100x100 mm). Five normal stresses of 11, 22, 33, 44 and 55 kPa and velocity of 0.2 mm / min were applied. Peak and residual shear stresses were evaluated (10% deformation of the nominal EPS length). The main results show a tendency to increase the interfacial friction with the increase of the specific mass of the EPS to the situation of peak stress. For the extreme conditions, with smaller and larger specific masses, the interface friction values ​​were, respectively, 18.27º and 31.77º for sandy soil. For clayey soil, these values ​​were 16.24 and 39.00. In contrast, both the interface friction analysis for the case of residual stress and the verification of the variation of this parameter between the peak and residual situations for each specific EPS mass, no characteristic behavioral trends were observed.