The invention relates to an apparatus for treating a medical liquid whichincludes atreatment machine having a coupling surface and a flexible mat arranged on thecoupling surface, with a cassette made of a hard part having liquid-conductingpassages which are covered by a flexible film being able to be coupled to thecoupling surface of the treatment machine via the flexible mat. In accordancewiththe invention, the apparatus for making possible the monitoring of a fault-freecoupling to the cassette at the coupling surface is made such that a pressuredifference of the pressure can be determined between the cassette and theflexiblemat and of the pressure between the mat and coupling surface. The inventionfurthermore relates to a method for the checking of the leak tightness of theaforesaid apparatus.
