Equilibria in the H{sup +}-Al{sup 3+}-MoO{sub 4}{sup 2{minus}} system have been studied by potentiometry and {sup 27}Al NMR spectroscopy in 0.6 M Na(Cl) medium at 25{degree}C. The total concentration of aluminum, B, and molybdate, C, were varied within the limits 0.9 {le} B/mM {le} 10, 5.5 {le} C/mM {le} 120, and 2 {le} C/B {le} 12 within the range 1.5 {le} -log (H{sup +}) {le} 6.0. All data unambiguously show that one single ternary species, Al(OH){sub 6}Mo{sub 6}O{sub 18}{sup 3{minus}}, is formed. According to potentiometric data, the formation constant for this species (as related to the reaction 6H{sup +} + Al{sup 3+} + 6MoO{sub 4}{sup 2{minus}} {r reversible} Al(OH){sub 6}Mo{sub 6}O{sub 18}{sup 3{minus}}) is log ({beta}{sub 6,1,6} {plus minus} 3{sigma}) = 50.95 {plus minus} 0.04. Independent {sup 27}Al NMR data yielded the value 50.96 {plus minus} 0.06, i.e. in full agreement with emf data. The high thermodynamic stability of the complex was manifested in a model calculation, showing that even at B = 10{sup {minus}5} M, C = 6 {times} 10{sup {minus}5} M, and -log (H{sup +}) = 3.7, 40% of B and C remained bound in this species. In acidic media, equilibration times of approximately 6 h were observed,more » while in near-neutral solutions the decomposition rate is higher by several orders of magnitude. With -log (H{sup +}) {le} 2, the complex decomposes with the formation of free aluminum ions and isopoly molybdates, while in near-neutral solutions (-log (H{sup +}) {ge} 5) the decomposition yields amorphous aluminum hydroxide and molybdate ions. The solubility product for this phase, defined by the reaction Al{sup 3+} + 3H{sub 2}O {r reversible} Al(OH){sub 3}(s) + 3H{sup +}, was determined to be log (K{sub {minus}3,1,0} {plus minus} 3{sigma}) = {minus}10.49 {plus minus} 0.08. Data were analyzed with the computer programs Letagropvrid and Solgaswater. 20 refs., 8 figs., 1 tab.« less