The reaction of H3[PW12O40] with (NH4)[NbO(C2O4)2(H2O)2]·3H2O leads to the straightforward incorporation of Nb into a Keggin anion backbone resulting in [PW11NbO40]4- in nearly quantitative yield. Solution behavior of this monosubstituted anion as a function of pH was studied through 31P NMR and HPLC-ICP-AES techniques (a combination of chromatography and ICP-AES). Increasing of the pH leads to the elimination of Nb from the structure with the formation of mixed Nb/W Lindqvist-type anions and monolacunary [PW11O39]7-. In aged solutions of [PW11NbO40]4- the formation of a mixture of [PNb2W10O39]5- isomers was detected. The HPLC-ICP-AES technique was applied for the study of [BW11O39]9- behavior in aqueous solutions at different pH values and for monitoring the [GeW11O39]4- reaction with the niobium oxalate complex. Despite the presence of the nucleophilic NbO group [PW11NbO40]4- does not coordinate Co2+ in DMF or Ni2+ in CH3CN, producing double [Co(DMF)6][PW11NbO40]·2.6DMF (2) and [Ni(CH3CN)5.5(H2O)0.5]2[PW11NbO40]·1.7CH3CN (3) salts instead.