A series of 1,4,7-triazacyclononane-based ligands containing one, two, or three methylphosphinate (MP) side chains has been prepared, The macrocycle with three methylphosphinates, NOTMP, appeared to be a very promising ligand for monitoring Mg-II in biological samples by P-31 NMR spectroscopy, The P-31 resonances of the free ligand and the Mg-II complex were well resolved, in slow exchange, and well downfield of typical tissue phosphate and phosphate ester metabolite resonances. The K-d value of the Mg(NOTMP) complex was 0.35 mM at physiological pH and temperature, a value which is optimal for accurate assessment of free Mg-II in cells and blood plasma. The latter was confirmed experimentally. Furthermore, the selectivity of NOTMP for Mg-II over Ca-II exceeded 2 orders of magnitude. The K-d values of the Mg-II complexes were sensitive to pH and temperature. The pH effects could be easily predicted from potentiometric pH titration data. Examination of the Mg-II ligand equilibria by P-31 NMR over a wide temperature range provided complexation enthalpies and entropies, It was shown that the smaller K-d values at higher temperatures were largely due to a temperature effect on the highest ligand protonation constant. Only a small endothermic complexation enthalpy contributed to this phenomenon.