The invention relates to a micromixer for mixing at least two fluids that react to form precipitations or suspensions. The micromixer comprises a first channel for supplying a first partial flow (6) and a second channel for supplying a second partial flow (7). These flows empty into a mixing and reaction area (10) via narrow entrance gaps (19, 20) and leave the mixing and reaction area (10) via an outlet channel (11). The invention is characterized in that a reverse flow prevention is placed between the mixing and reaction zone (10) and at least one channel that supplies a partial flow (6, 7, 37).