The Bi-Se system was studied by differential thermal analysis, powder X-ray diffraction, scanning electron microscopy, as well as by electromotive forces (EMF) measurements of electrochemical cells (−) Bi (solid) │ionic liquid + Bi 3+ │ Bi-Se (solid) ( +). A new refined version of the phase diagram reflecting the compounds Bi 2 Se 3 , Bi 3 Se 4 , Bi 8 Se 9 , BiSe, Bi 8 Se 7 , Bi 4 Se 3 , and Bi 3 Se 2 with almost constant composition was constructed. It was established that the first compound melts congruently, and all the rest melt with peritectic reaction. The types and parameters of the crystal lattices of the above-mentioned compounds were determined based on the powder diffraction patterns. From the EMF measurements, the partial molar functions of bismuth in alloys and the standard integral thermodynamic functions of bismuth selenides were calculated. Comparative analysis of the obtained results with the literature data is carried out.