The fluid flow and the mixing process in a static mixer have been analysed to gain insight into the character of the mixing process and to check the numerical simulation as a tool for the design of single mixers and entire facilities with mixers incorporated. In a first step the mixing process in a straight pipe has been analysed by a numerical simulation, and good agreement has been found with experimental data. The investigation has been extended to an infinite, ideal mixer element. The vortices generated by the structure of the mixer have been observed to be the main driving force of the mixing of the fluid. This is the reason why the main mixing occurs in the wake of the mixer. As an industrial application the fluid flow and the mixing in the inlet part of a denitrification facility has been computed. it has been found that mixing quality is improved significantly by incorporating a static mixer. The measured velocity and concentration data also agree well with the numerically obtained results.