A nonstoichiometric Nb 12 O 29 (NbO 2.417 ) block structure oxide with monoclinic symmetry was slightly oxidized in a controlled environmental high-resolution electron microscope. This instrument is based on a JEOL 4000 EX electron microscope equipped with a unique gas reaction cell in which the oxide could be oxidized by introducing O 2 at pressures up to 30 mbar, while promoting local reaction by electron beam heating. The oxidation process was seen to start in the regions where some stacking faults perpendicular to the c axis appear (in which the structure has local orthorhombic symmetry). The initial movement of atoms, involving a formation of lamellar defects, could be traced with great certainty. Such defects provide empty rectangular tunnels, similar to those found in the NaNb 13 O 33 structure, which is a (4؋3) block structure with Na cations situated in the rectangular tunnels. This stage of the oxidation reaction implies a minimal rearrangement, usually involving a simple unit jump (the primitive di4usion step) by atoms in certain sites. Then, the cooperative unit jumps along rows of atoms propagate the transformation through the crystal. A further stage in the oxidation gives rise to the Nb 22 O 54 (NbO 2.455 ) structure in some areas of the crystal, which could be identi5ed in the high-resolution transmission electron microscopy image as well as in the electron di4raction patterns.