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The totally symmetric stretching frequencies u5(Si-X) of the octasilases-quioxanes X8Si8O12are distinctively larger than those of XSi(OSiMe3)3for X = H, CH3, Cl whereas u5(Si-H) of H8Si8O12and of H10Si10O15differ very little. Inspection of the experimental data and molecular orbital calculations of the EHMO-ASED type show that this striking discrepancy is caused by the differences in the X-Si-O-Si conformations. The cage structure of X2Si8O12requires the anti X-Si-O-Si conformation whereas the syn conformation is the stable one in XSi(OSiMe3)3. The Si-H bond order decreases from anti to syn caused by a decreasing interaction of the H-ls orbital with the Si-O-pπ type orbitals.