Raman spectroscopy was employed to investigate the KNO3-RbNO3 system. Raman studies indicated that Rb+ may substitute K+ up to 67 mol % in the solid solutions of the KNO3 II structure (denoted as K1-xRbxNO3 (KII)) and up to 80 mol % in the solid solutions of the KNO3 III structure (denoted as K1-xRbxNO3 (KIII)). The substitutional crystals retained the transitions of I to III to II of KNO3 on cooling and the metastable property of KNO3 III at room temperature. It was found that rubidium nitrate had a considerable tendency to have the structure of potassium nitrate. This accounts for the fact that larger rubidium ions can replace many more smaller potassium ions in the nitrate than vice versa. When the concentration of rubidium ions was more than 67 mol %, the substituted crystals underwent the mixed structural phase transition of the KNO3 III structure to the RbNO3 IV structure, and K1-xRbxNO3 (KIII) seemed to consist of disordered R3m microstructure.