Phase equilibria in the silicon-rich part of the system Si+Yb ( > 60 at.% Si) have been established from DTA, LOM, EMPA, and X-ray di4raction experiments on arc-melted and annealed bulk alloys as well as on single crystals grown from pure gallium or indium metal used as a 6ux solvent. Phase relations are characterized by the existence of a defect disilicide showing polymorphism. Yb 3 Si 5 , the low-temperature modi5cation with Th 3 Pd 5 type, is a line compound at 62.5 at.% Si, stable below 947 ؎ 73C. Above this temperature Yb 3 Si 5 transforms into the YbSi 2؊x (defect AlB 2 -derivative type) corresponding to a peritectoid equilibrium at 947 ؎ 73C: YbSi 2؊x ؉ YbSi 0 0 Yb 3 Si 5 . YbSi 2؊x exhibits a small homogeneity region from &62.5 at.% to &63.5 at.% Si and melts incongruently at 1408 ؎ 93C at &63 at.% Si. On cooling it decomposes according to a eutectoid reaction at 763 ؎ 73C: YbSi 2؊x 0 0 Yb 3 Si 5 ؉ (Si). The silicon-rich part of the diagram is characterized by a eutectic equilibrium at 1135 ؉ 73C and &81 at.% Si: L 0 0 YbSi 2؊x ؉ (Si). From magnetic susceptibility and electrical resistivity measurements, performed on single-crystalline specimens, Yb 3 Si 5 was found to be an intermediate valent system.