Diethylenetriamine-N,N,N',N '',N ''-pentakis(methylenephosphonic acid) (DTPMPA) is used in a wide range of industrial applications, mainly because of its binding ability toward several metal cations. Because of difficulties in its synthesis and purification, very little reliable data have been reported in the literature about the coordination chemistry of this ligand in aqueous solution. For these reasons, in this article, we report an efficient procedure for the synthesis and purification of DTPMPA. The pure product obtained was used to determine its acid-base properties in different aqueous ionic media, namely, (C2H5)(4)NI(aq), NaCl(aq), and KCl(aq), at 288.15 <= T/K <= 318.15 [only T = 298.15 K for KCl(aq)] by potentiometry (H+ ion-selective electrode, glass electrode) and at different ionic strengths (0 < I/mol L-1 <= 1.0). Measurements performed in alkali metal chlorides were also interpreted in terms of weak complex formation between DTPMPA and Na+ and K+, and further measurements were also performed in NaCl(aq) at T = 298.15 K and different ionic strengths (0 < I/mol L-1 <= 1.0) in the presence of Mg2+ or Ca2+, to determine the stability constants of species formed by DTPMPA and these cations. The protonation and complex-formation constants obtained at different ionic strengths and temperatures were modeled by different equations, providing all of the thermodynamic data necessary to define the solution behavior and the chemical speciation of DTPMPA under a wide number of variable conditions, such as those encountered in the very different industrial applications in which this chelating agent is used and those involving many natural fluids.