Reaction bonded MgSiN2 (RBMSN) was prepared by direct nitridation of a Si/Mg2Si/Mg/Si3N4 powder compact in a temperature range of 1350-1550°C. The oxygen content of MgSiN2 was in the range of 0.4 – 0.6 wt%. A thermal stability examination showed that MgSiN2 is stable up to 1400°C at 0.1 MPa N2 pressure. The activation energy of decomposition calculated from the temperature dependence of weight loss is H = 383 kJ⋅mol-1. The time dependence and nitrogen pressure dependence of MgSiN2 decomposition was also investigated at constant temperature. MgSiN2 is stable at 1560°C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs free energy of formation of MgSiN2 was calculated in a temperature range 300-2500 K. Dense MgSiN2 ceramics or MgSiN2/Si3N4 composites with fluorine-based additives were prepared by hot pressing. The composite materials had a 4-point bending strength of 427 MPa and Vickers hardness (HV1) of 20.8 GPa, respectively. The indentation fracture toughness was 5.3 MPa.m1/2, due to the presence of elongated β-Si3N4 grains. The dielectric constant of dense reaction bonded MgSiN2 at 100 kHz was 9.5-10, while that of MgSiN2/Si3N4 composite in a wide range 50 – 6000, depending on composition and heat treatment.