Mixed Nb-Mo oxides were prepared by solid-state reaction of Nb(V) and Mo(VI) oxides at 973-1123 K. Optimal conditions were determined for the formation of the Mo3Nb2O14 compound. As established by Rietveld refinement of the powder X-ray diffraction patterns, the Mo3Nb2O14 oxide has the tetragonal cell with a = 23.150(6) angstrom and c = 3.998(4) angstrom and a tunnel structure similar to that of the Mo5O14 oxide. The solids were characterized by several physical techniques, including scanning and transmission electron microscopy, FTIR, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance spectroscopy. It has been shown that the Mo3Nb2O14 solid prepared in air at 973-1073 K after cooling to room temperature contains high amounts of Mo(V) species (ca. 1% of total molybdenum). The presence of paramagnetic species correlates with the intense green color of the solids and the strong d-d transition band in the UV-visible spectra, typical for the d(1) species. The amount of paramagnetic species does not depend on the solid annealing and/or on the small variations of its composition. Neither is it related to the oxygen release upon the solid heating, being therefore an intrinsic property of the Mo3Nb2O14 oxide. The unusual stabilization of reduced Mo species in the highly oxidizing conditions was explained by the substitution disorder between Nb and Mo atoms. It is supposed that a configuration containing mu(3) oxygen bonded to three Mo(VI) atoms is unstable and decomposes, leading to a Mo(V) center and a hole in the valence band.