The interaction of the protonated forms of tris(2-aminoethyl)amine (tren) with NO3-, SO42-, TsO-, PO43-, P2O74-, and P3O105 was studied by means of potentiometric and microcalorimetric measurements in a 0.10 M NMe4CI aqueous solution at 298.1 +/- 0.1 K, affording stability constants and the relevant energetic terms Delta H degrees and T Delta S degrees of complexation. Thermodynamic data show that these anion complexation processes are mainly controlled by electrostatic forces, although hydrogen-bond interactions and solvation effects also contribute to complex stability, leading, in some cases, to special Delta H degrees and T Delta S degrees contributions. The crystal structures of [H3L][NO3](3) and [H3L][TSO](3) evidence a preferred tridentate coordination mode of the triprotonated ligands in the solid state. Accordingly, the H3L3+ receptor binds a single oxygen atom of both NO3- and TsO by means of its three protonated fingers, although in the crystal structure of [H3L][TsO](3), one conformer displaying bidentate coordination was also found. Modeling studies performed on the [H3L(NO3)](2+) complex suggested that the tridentate binding mode is the preferred one in aqueous solution, while in the gas phase, a different complex conformation in which the receptor interacts with all three oxygen atoms of NO3- is more stable.