The hydrolysis of trimetaphosphate (P-3m) is catalyzed dramatically by La(NTA) (NTA : nitrilotriacetate), but less pronounced by La(EDTA) (EDTA : ethylenediaminetetraacetate). Reaction products are tripolyphosphate (PPP), diphosphate (PP), and monophosphate (P). A multinuclear magnetic resonance study shows that there are only marginal differences between La(NTA)(P-3m) and La(EDTA)(P-3m) regarding complex geometry and P-O bond activation of P-3m. The affinity of P-3m is lower for Ln(EDTA) than for Ln(NTA). The catalyzed hydrolysis probably leads to tridentate coordinated PPP, after which ligand rearrangement takes place. Complexation of PPP to Ln(NTA) or Ln(EDTA), as also reflected in the formation constants, caused decoordination of one carboxylate group of NTA and two of EDTA. The gain in free energy, which is larger for Ln(NTA) than for Ln(EDTA), going from complexed P-3m to PPP may be a driving force of the reaction. Another important factor in this respect is the much higher stability of La(NTA)(OH) compared to that of La(EDTA)(OH). This leads to substantially higher concentrations of reactive complex La(L(1))(P-3m)(OH) for L(1) = NTA, from which it is concluded that the hydrolysis of P-3m catalyzed by La(NTA) proceeds via attack of coordinated hydroxide onto coordinated P-3m. The reactions catalyzed by La(EDTA) are probably initiated by both free and coordinated hydroxide. A kinetic model is developed, in which stabilities and rate constants are incorporated, to predict the speciation during the hydrolysis. Because of the fast subsequent hydrolysis of PPP and PP, competition between P-3m and P for complexation to La(III) is the most important factor governing the inhibition observed during the hydrolysis of P-3m at prolonged reaction times.