The sol-gel process allows the high throughput formation of transition metal oxide thin films. Microwave plasma annealing (MwPA) treatments have been performed on thin films of two different transition metal oxides, Ta2O5 and ZnO, selected as representatives of covalently and strongly ionic bonded oxides, respectively. Ta2O5 has been explored as a dielectric barrier for porous silicon structures. The main limitation of the sol-gel spin coating in this case is the surface roughness of the coating, which is highly improved upon Ar MwPA. The treatment leads additionally to a microstructural activation and interface development comparable to a 500ºC thermal annealing. The MwPA of ZnO is a quasi-equivalent process to a 200ºC thermal annealing, preventing grain growth and promoting nanocrystalline phases. This is suggested to have a direct impact on the optical and electronic properties of the ZnO films. The MwPA films show wider optical band gap than thermally annealed ones. An impedance analysis further shows that the MwPA ZnO films present lower equivalent resistance and higher equivalent capacitance than the thermal films. These results are promising for the development of new processing routes for widely demanded transition metal oxide thin films.