MoVTeNbO catalysts with several Mo/Te/V/Nb (1/0.15-0.7/0-0.5/0-0.9) contents have been prepared by hydrothermal synthesis and tested in the selective oxidation of propane and propene to acrylic acid. Characterization results (XRD, FTIR, SEM-EDX, and XPS) and catalytic tests show important differences, depending on the composition of the catalysts. In this way, several crystalline phases have been observed in the active and selective catalysts, i.e., TeMo5O16, (Mo0.93V0.07)(5)O-14, 3MoO(2)Nb(2)O(5), and/or Nb0.09Mo0.91O2.80, and a new TeVMo oxide crystalline phase. Vanadium is the key element in the activation of propane and the selective achievement of acrylic acid while V- and/or Nb-doped MoTe-containing crystalline phases are related to the selective transformation of propene to acrolein/acrylic acid. However, the role of Nb ions is still unclear. Nb-containing MoVTe catalysts present both high activity and high selectivity to acrylic acid. Space time yields of acrylic acid closer than 70 and 600 g(AA) Kg(cat)(-1) h(-1) can be obtained at 380degreesC during the oxidation of propane (keeping selectivities of about 55%) and propene (keeping selectivities of about 80%), respectively.