The stoichiometries and stability constants of a series of Al 3+ –N-phosponomethyl glycine (PMG/H 3 L) complexes have been determined in acidic aqueous solution using a combination of precise potentiometric titration data, quantitative 27 Al and 31 P NMR spectra, ATR–FTIR spectrum and ESI-MS measurements (0.6M NaCl, 25°C). Besides the mononuclear AlH 2 L 2+ , Al(H 2 L)(HL), Al(HL)2- and Al(HL)L 2− , dimeric Al 2 (HL)L + and trinuclear Al3H5L42+ complexes have been postulated. 1 H and 31 P NMR data show that different isomers co-exist in solution and the isomerization reactions are slow on the 31 P NMR time scale. The geometries of monomeric and dimeric complexes likely double hydroxo bridged and double phosphonate bridged isomers have been optimized using DFT ab initio calculations starting from rational structural proposals. Energy calculations using the PCM solvation method also support the co-existence of isomers in solutions.