The cluster aqua complexes [Mo 3 (MCl)Q 4 (H 2 O) 9 ] 3+ (M = Pd or Ni) in hydrochloric acid solutions induce isomerization of the hydrophosphoryl compounds (HO) 2 P(O)H, (HO)P(O)H 2 , PhP(O)(OH)H, and Ph 2 P(O)H into the hydroxo tautomers P(OH) 3 , HP(OH) 2 , PhP(OH) 2 , and Ph 2 P(OH), which are stabilized by coordination of the phosphorus atom to the Pd or Ni atoms. The reactions were studied by 31 P NMR and UV Vis spectroscopy. The kinetics of the reactions of phosphorous acid with [Mo 3 (PdCl)Q 4 (H 2 O) 9 ] 3+ was investigated by spectropho tometry. The [Mo 3 (Pd(PhP(OH) 2 ))S 4 (H 2 O) 2 Cl 7 ] 3-complex was isolated as a supramolecular adduct with cucurbit[8]uril, and the [Mo 3 (Ni(P(OH) 3 ))S 4 (H 2 O) 8 Cl] 3+ complex was isolated as an adduct with cucurbit[6]uril. The structures of both compounds were established by X ray diffraction analysis.