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 31P and 183W NMR in solution as well as by 31P 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.