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.