ABSTRACT The small-strain stress-strain properties of expanded polystyrene (EPS) geofoam with densities of about 20 kg/m3 and 30 kg/m3 were evaluated by laboratory unconfined compression tests on specimens of 75 mm in diameter and 150 mm in height. Two series of tests were conducted, which were continuous monotonic loading (ML) tests and ML tests intervened by sustained creep loading and minute cycles of unload and reload. Relatively small vertical and horizontal strains were locally measured by means of a pair of local deformation transducers (LDTs) and a set of three clip gauges, respectively. The paramount importance of measuring local strains in compression tests on EPS to reliably evaluate its stress-strain properties, in particular those at relatively small strains, is demonstrated. The initial modulus, E0, and Poisson's ratio, ν0, were evaluated from initial stress-strain relations at small strains obtained by these ML tests. The tangent parameters, Etan and νtan, were also evaluated from the ML stress-strain behaviour. The equivalent parameters, Eeq and νeq, were evaluated from the stress-strain behaviour during minute cycles of unload and reload. The stress-strain behaviour is essentially linear only at small strains, and it becomes highly non-linear and a significant drop of stiffness occurs as observed in the overall stress-strain behaviour. The Poisson's ratio for inelastic deformation is found to be negative.