Available thermodynamic and phase diagram data have been combined with estimation, modeling and calculation techniques to develop a large thermodynamic database for refractory boride, carbide, nitride and silicide systems. The work has been facilitated by the computer code due to Gunnar Eriksson, which is incorporated in the FactSage software. The database provides thermodynamic parameters for very many previously un-assessed systems based on alloys of the elements Al, Co, Cr, Fe, Hf, Mn, Mo, Nb, Ni, Re, Sc, Ta, Tc, Ti, V, W, Y, and Zr with C, B, N and Si. Calculations of thermodynamic properties and phase diagrams can be carried out with reasonable to good reliability for approximately 250 binary, and about 330 ternary systems, for the individual temperatures or temperature ranges covered by the available experimental information. Comparison is made of calculated and experimental thermodynamic values and phase equilibria for a number of representative systems.