This paper summarizes preparations of mixed phosphoniobotungstates [PNbxW12-xO40]((3+x)-) (x = 1-3) in two different ways: (i) incorporation of Nb into pre-existing lacunary phosphotungstates and (ii) self-assembly reactions. A niobium oxalate complex as the source of niobium is used. The reaction products were monitored by P-31 and W-183 NMR in solution as well as by P-31 MAS NMR techniques for precipitated samples. Single-substituted species [PNbW11O40](4-) forms rapidly and selectively from lacunary [PW11O39](7-) with one equivalent of [NbO(C2O4)(2)(H2O)(2)](-). With the increasing amount of the niobium oxalate the highest number of Nb atoms was found to be three per Keggin anion, but the disubstituted [PNb2W10O40](5-) was always the preferable product whatever the tungsten reagent was. The reaction products with x = 2 and 3 are present as mixtures of all possible isomers, but the ratio between the individual isomers depends on the synthetic pathway.