The potentiometric titration of a gamma-Keggin tetra-protonated silicodecatungstate, [y-SiW10O34(H2O)(2)](4-) (H-4.O), with TBAOH (TBA = [(n-C4H9)(4)N](+)) showed inflection points at 2 and 3 equiv of TBAOH. The H-1, Si-29, and W-183 NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [gamma- SiW10O34(OH) (OH2)(5-) (H3.I) [gamma-Siw(10)O(34) (OH)(2)](6-) (H2.I), and [gamma-SiW10O35(OH)](7-) (H.I), with C-1, C-2 and C-2 symmetries, respectively. Single crystals of TBA(6).H-2.I suitable for the X-ray structure analysis were successfully obtained and the anion part was a monomeric gamma-Keggin divacant silicodecatungstate with two protonated bridging oxygen atoms. Compounds H-3.I, H-2.I, and H.I were reversibly monoprotonated to form H-4.I, H-3.I, and H-2.I, respectively.