The electronic properties, redox properties, protonation, and stability of five [alpha-PTi2W10O40](7-) isomers have been investigated employing density-functional theory (DFT) method. The results reveal that the stability of [alpha-1,2PTi(2)W(10)O(40)](7-) and [alpha-1,6-PTi2W10O40](7-) is weaker and the redox ability is higher among five isomers, while for [alpha-1,5-PTi2W10O40](7-), [alpha-1,4-PTi2W10O40](7-), and [alpha-1,11-PTi2W10O40](7-) the stability is higher, but the redox ability is weaker. At the same time, Ti-substituted systems are preferentially reduced in the tungsten centers. A simple analysis of molecular electrostatic potential maps of [alpha-PTi2W10O40](7-) isomers suggests that the preferred protonation site corresponds to bridging oxygens (OTi2 and OTiW) and terminal oxygens (OTi), especially bridging oxygens bonded to titaniums (OTi2) in [alpha-1,2-PTi2W10O40](7-) and [alpha-PTi2W10O40](7-). It is proposed that the most stable structure is formed preferentially after protonation of the Ti-O site from the study of the protonated species, [alpha-1,4-HPTi2W10O40](6-). By means of total bonding energies of [alpha-PTi2W10O40](7-) isomers, the relative sequence of stability has then been shown to be [alpha-1,4-PTi2W10O40](7-) > [alpha-1,5-PTi2W10O40](7-) > [alpha-1,11-PTi2W10O40](7-) > [alpha-1,2PTi(2)W(10)O(40)](7-) > [alpha-1,6-PTi2W10O40](7-). In addition, the one-electron-reduced species of [alpha-PTi2W10O40](7-) are also discussed.