A series of isotopologues of the siloxanediol Mes2Si(OH)(µ-O)Si(OH)Mes2 (3a) (Mes = 1,3,5-trimethylphenyl) were synthesized by reactions of the corresponding disiloxane precursors Mes2Si(µ-O)2SiMes2 (2a), Mes2Si(µ-17O)2SiMes2 (2b) or Mes2Si(µ-18O)2SiMes2 (2c) with an excess of H2O, H217O or H218O. NMR and IR signal assignments for the siloxanediols in benzene are supported by quantum-chemical calculations, which indicate small energy differences between trans and cis conformers, the latter of which exhibits an intramolecular hydrogen bond. 1H NMR as well as IR data suggest the presence of a mixture of both conformers in C6D6. Hydrogen-bonded adducts of Mes2Si(OH)(µ-O)Si(OH)Mes2 with ethers such as diethylether, dimethoxyethane or dioxane were observed in the solid state, where they form polymeric chain-like structures. The latter appears to be stable only in the crystal. 17O{1H} NMR and IR data in THF solution suggest an interaction of 3a with at least one THF molecule, whereas diethylether appears not to interact. Water adducts form neither in solution nor in the solid state as indicated by NMR and ATR IR data. 17O{1H} NMR and ESI-MS experiments illustrate the remarkably high stability of the siloxanediols towards water and show no evidence for intra- or intermolecular oxygen-exchange reactions. In marked contrast, a stepwise exchange of all three oxygen atoms – including the one in the Si-O-Si bridge – occurred in the gas phase, when [Mes2Si(18OH)(µ-18O)Si(18O)Mes2]− was treated with H2O in the hexapole of an ESI FT-ICR mass spectrometer. The scrambling between the bridging and the other oxygen atoms likely proceeds through cyclic Si2O2 intermediates.