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Chemical reactions and phase stabilities in the Si-Te system at high pressures were explored using in situ angle-dispersive synchrotron powder diffraction in a large-volume multianvil press together with density functional theory-based calculations. Cubic and rhombohedrally distorted clathrates, with the general formula Te 8 @(Si 38 Te 8 ) and wide compositional range, preceded by a hexagonal phase with the composition Si 0.14 Te, are formed for different mixtures of Si and Te as starting materials. Si 0.14 Te, with the structural formula Te 2 (Te 0.74 Si 0.26 ) 3 (Te 0.94 Si 0.06 ) 3 , is the very first chalcogenide with the Mn 5 Si 3 -type structure. Silicon sesquitelluride α-Si 2 Te 3 decomposes into a mixture of phases that includes the clathrate and hexagonal phases at high pressures and high temperatures. The higher the pressure, the lower the temperature for the two phases to occur. Regardless of the starting compositions, only the clathrate is quenched to atmospheric conditions, while the hexagonal phase amorphizes on decompression. The rhombohedral clathrates Te 8 @(Si 38 Te 8 ) form on quenching of the cubic phases to ambient conditions. There is a high degree of interchangeability of Si and Te not only in the clathrates but also in the Mn 5 Si 3 -type structure. The theoretical calculations of enthalpies indicate that the reported decomposition of α-Si 2 Te 3 is energetically favorable over its transformation to another polymorph of the A 2 X 3 type at extreme conditions.