An exhaust gas discharged from a furnace for producing polycrystalline silicon from a chlorosilane and hydrogen is condensed to separate the hydrogen. The resultant condensate is distilled to separate the unreacted chlorosilane and by-product silicon tetrachloride and is then further distilled to recover disilicon hexachloride. Tetrachlorodisilane can be recovered together with the disilicon hexachloride. The disilicon hexachloride and tetrachlorodisilane recovered have a far higher purity than the conventional ones produced from metallic silicon.
