The electronic properties, redox properties, protonation, and stability of five [alpha-PTi(2)W(10)O(40)](7)(-) isomers have been investigated employing density-functional theory (DFT) method. The results reveal that the stability of [alpha-1,2-PTi(2)W(10)O(40)](7)(-) and [alpha-1,6-PTi(2)W(10)O(40)](7)(-) is weaker and the redox ability is higher among five isomers, while for [alpha-1,5-PTi(2)W(10)O(40)](7-), [alpha-1,4-PTi(2)W(10)O(40)](7)(-), and [alpha-1,11-PTi(2)W(10)O(40)](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-PTi(2)W(10)O(40)](7)(-) isomers suggests that the preferred protonation site corresponds to bridging oxygens (OTi(2) and OTiW) and terminal oxygens (OTi), especially bridging oxygens bonded to titaniums (OTi(2)) in [alpha-1,2-PTi(2)W(10)O(40)](7)(-) and [alpha-PTi(2)W(10)O(40)](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-HPTi(2)W(10)O(40)](6)(-). By means of total bonding energies of [alpha-PTi(2)W(10)O(40)](7)(-) isomers, the relative sequence of stability has then been shown to be [alpha-1,4-PTi(2)W(10)O(40)](7)(-) > [alpha-1,5-PTi(2)W(10)O(40)](7)(-) > [alpha-1,11-PTi(2)W(10)O(40)](7)(-) > [alpha-1,2-PTi(2)W(10)O(40)](7)(-) > [alpha-1,6-PTi(2)W(10)O(40)](7)(-). In addition, the one-electron-reduced species of [alpha-PTi(2)W(10)O(40)](7)(-) are also discussed.