A novel annular catalytic reactor was experimentally and theoretically characterized. CO combustion was adopted as test reaction to probe the reactor behavior under chemical and under diffusion controlled regimes. It was found that mass transfer is sensitive to the eccentricity of the annular duct. The analysis evidenced that much higher space velocities and reaction temperatures, as well as better isothermal conditions can be guaranteed by the present annular reactor in comparison with standard fixed-bed reactors.