There is described a microfluid system with a microchannel structure for the passage of fluids and a further channel structure separated from the microstructure by means of at least one separating wall for the passage of a heat transfer fluid. The risk of internal leakage in the microfluid system can be recognized in time, by separation of the microchannel structure from a cavity of at least one point by a further separating wall, said separating wall being at least locally weaker than the separating wall between the microchannel structure and the further channel structure and said cavity is connected to a detector device for detection of ingressing fluid.