We report a molecular dynamics study on pertechnetic acid (PTA) extraction from water to an oil phase containing either pure TBP (tri-n-butyl phosphate) or a TBP/n-hexane mixture, with the main aim to understand the state of the acid (associated TcO4H vs dissociated TcO4-H+) and its "complexation" by TBP. Experimentally, Tc-VII is extracted from acidic water to TBP:alkane solutions in 1:3 or 1:4 Tc:TBP ratio, suggesting that three or four TBPs coordinate to TcO4H or TcO4-. According to simulations in TBP solution, however, neither TcO4H nor TcO4- species displays specific coordination to TBP. We thus investigated several hypothetical states of the proton of the dissociated pertechnetic acid in organic phases and at their aqueous interfaces, comparing "pH neutral" to nitric acid containing systems. Proton hydrates are shown to coordinate 3-4 TBPs, in the form of H3O+ (TBP)(3) and H5O+2 (TBP)(4) hydrogen-bonded adducts, whereas TBPH+ binds 1 TBP. The MD and PMF results complemented by those of QM investigations suggest that Tc-VII is extracted as TcO4-(H3O)(+)(TBP)(3) or TcO4-(H5O2)(+)(TBP)(4) contact ion pairs instead of the neutral form TcO4H of the acid. They explain why nitric acid promotes the Tc-VII extraction. Comparison between nitric acid (mainly extracted via its neutral form NO3H) and pertechnetic acid is discussed.