Two methods for uniaxial compression testing were investigated and compared using polypropylene as a model material. An overview of various parameters affecting compression test results is provided with particular emphasis on friction between the specimen and the compression plate. A procedure for the determination of the compressive modulus is introduced and results are shown. To enable the detection of instability associated with friction and barreling and to calculate true stress-true strain curves, the measurement of transverse expansion of the specimen at large strains is necessary. Nominal and true Poisson's ratio values in the pre- and post-yield regime are presented and the resulting true stress-true strain curves are compared and discussed. While in the post-yield regime nominal stress values misleadingly result in strain hardening, significant strain softening was observed using true stress values representing the intrinsic material behavior.