Mg-Zn-Sn alloys exhibit poor structural stability at elevated temperatures that restricts utilization of these alloys. Small additions of alloying elements forming high temperature phases (HTP) were used to improve the structural stability of the Mg-Zn-Sn alloy. The main goal of this work was to investigate the microstructure evolution of the Mg-Zn-Sn-alloy with additions of Y and Sb during a wide scope of heat treatments, and to elucidate peculiarities of an HTP-stabilized microstructure. In order to clarify the substructure features and phase precipitation after each step of the heat treatment, XRD, TEM, SEM and EDS analyses were applied. It was found that in the dendrite structure formed during solidification, HTP-particles are concentrated in the inter-dendrite regions. Solution treatment of the as-cast structure at 440 degrees C for 96 h lead to the formation of alpha-Mg grains of 50-80 mu m in diameter with a characteristic substructure. The presence of HTP-particles prevented dislocation recovery and movement of dislocation walls during solution treatment, and by this way restricted annihilation of grain boundaries between dendrites of close orientation, and lead to the formation of a substructure with sub-grains of 20-30 mu m. The sub-grain boundaries are pinned by HTP-particles and are strengthened by the MgZn2 and Mg2Sn binary precipitates during aging. Precipitate depleted zones formed near grain- and sub-grain boundaries during aging were bordered by a "crust" of enlarged binary particles. Such pinned sub-grain microstructure provides a high structural stability of the alloys at elevated temperatures.