Electron microscopic observations revealed the development of a complex interpenetrating morphology of magnesia and tantalum after magnesium-vapour reduction of tantalum oxide powder. In the early reaction stage, a nanoscale lamellar and rod-like morphology is found. This rather regular structure decomposes and coarsens during further annealing. By leaching of magnesia, nanoporous tantalum with large inner surface area is obtained. A possible growth mechanism for the observed initial cellular structure is proposed. A theoretical analysis aims at finding the optimum process parameters in order to prepare and to stabilise especially fine magnesia-tantalum structures as a base for producing highly nanoporous tantalum for capacitor applications.