The geometries of the hydronium and dihydroxonium cations in ion pairs with fluoroborate anions were examined by ab initio calculations at the MP2/6-31G* level. It was found that the representation of the hydronium ion in the field of an anion as an equilateral triangle, employed in the literature for the interpretation of low-temperature broad-band NMR spectra of water in solid acids,: Is an oversimplification, particularly for the composition H5O2+ (dihydroxonium). Chemical shift calculations (DFT-GIAO-B3LYP at the dzvp, tzp, tz2p, and qz2p levels) were conducted for O-17 in H3O+. BF4- (1) and H5O2+. BF4- (2). The signal of 2 was predicted to appear at higher frequency (downfield) than the signal For 1. For experimental verification, the O-17 NMR spectra were recorded for various mixtures of hydronium fluoroborate and water. A nonmonotonic variation of the O-17 chemical shift with the increase in water content was observed: the signal moved first toward higher frequency and had the highest chemical shift for a water-to-hydronium ratio of 1:1 (H5O2+). after which a monotonic variation toward lower frequency (upfield) was observed. Along both branches of the delta O-17 VS composition plot (H3O+. BF4-to H5O2+. BF4- and H5O2+. BF4- to H2O) the chemical shift variation was nonlinear. Thus, the experiments and the calculations were in qualitative agreement (the signal for 1 at lower frequency than the signal for 2), but the chemical shift difference predicted by the calculations was larger than the experimental result. Better agreement between the calculated and measured chemical shift differences is obtained for an orientation of ions in 2 with two fluorine atoms hydrogen bonded with the cation. Likewise. a better agreement is obtained for the pyramidal form than for the planar form of 1, in agreement with the geometry optimization results.