In relation to the time-scale of chemical kinetics, diffusive transport in micro-devices is faster than in conventional mixers. To exploit the resulting potential for chemical process engineering, size effects evident in the transport processes have to be understood. For this purpose, the scaling behaviour concerning the transport of mass, momentum and heat are considered. Just as much, the mixing behaviour of flow mixers on micro-scales needs to be further investigated. Therefore, based on numerical simulations, the mixing characteristic of a T-shaped micro-reactor with rectangular cross sections is studied for three different flow regimes. For the description of the mixing quality, Danckwerts’ intensity of mixing is complemented by a measure of the scale of segregation which employs the concept of specific contact area. To assess the efficiency of mixing in ducts, the cross directional contribution of the energy dissipation rate is defined. These concepts are applied to a T-shaped micro-mixer, employing high-resolution CFD-simulations. Furthermore, comparison with given experimental data is performed and shows remarkable agreement.