BET-model parameters for the binary system Mn(NO3 )2-H2 O were obtained by fitting experimental water activities at 298.15 K. The values obtained in this work were compared to those fitted previously for the systems Ca(NO3)2-H2O and Zn(NO3)2-H2O, showing reasonable agreement. With the model parameters fitted at 298 K,the vapor pressure of the saturated solution for the phases Mn(NO3 )2 · 6 H2 O and Mn(NO3 )2 · 4 H2 O were predicted and shows quite good agreement with the experimental values. The phase diagram of the Mn(NO2 )2-H2O system reproduced by the BET model shows smaller deviation from the experimental data than that by the extended UNIQUAC model. The ternary phase diagrams of the systems Me(NO3)n-Mn(NO3)2-H2O (Me= Mg, Ca and Li) are predicted with the binary model parameter and compared with available experimental data. The predicted eutectic compositions were given as possible heat storage materials.