The vapor composition and thermodynamic properties of Si-B alloys with boron content from 1.5 up to 100 at.% were investigated in the temperature interval of 1522 to 1880 K by Knudsen effusion mass spectrometry. Thermodynamic functions of the SiB6 and SiBn borides, primary solid solutions, and liquid solution were obtained. The thermodynamic functions of the Si-B melt were approximated by the ideal associated-solutions model under the assumption that only one complex, SiB3, existed. The established thermodynamic functions of the melt and of the SiB6 and SiBn compounds were used for computation of the phase diagram of the Si-B system. Good agreement with the available experimental data was obtained.