There is a great interest in the last years in the study of Mg-Y-Zn alloys containing LPSO phases. This kind of alloys behaves as a metal matrix composite where the LPSO phase acts as the reinforcement with appreciate ductility. However, the plastic deformation of this kind of alloys is still not clear understood. The evolution of the internal strains in both phases, magnesium matrix and LPSO, during in situ tensile and compressive experiments (at room temperature and 250 ºC) will clarify the deformation system that takes place during plasticity and the reinforcement capacity of the LPSO phase. During compressive test, the activation of twinning will be also evaluated.