Abstract To demonstrate and to extend the performance of acoustic emission testing as a method of detecting and classifying flaws, six institutes conducted acoustic emission measurements in the course of various loading tests on a medium-sized, thickwalled vessel (model of a reactor pressure vessel) containing natural flaws. This paper will present an outline of the working program of the MPA within the framework of this project. The presentation is concentrated on the vessel manufacturing, the preparation of the flaw patch with 14 natural flaws, the performance of the loading tests to simulate pressure test and operating conditions existing in the primary systems of pressurized water reactors, and especially on the conduction of acoustic emission measurements as a method of permanent vessel monitoring during the tests. In the pressure tests conducted with slowly rising pressures, only one flaw was detected unequivocally by the acoustic emission monitoring, although several flaws had grown in the test phases between the pressure tests. In fast pressure tests, flaws in principle were detected slightly better. Cyclic loading over prolonged periods of time produced clear signals of larger flaws, which calculations and subsequent destructive investigations showed to have grown. The small flaws which, most probably, had not changed, could not be detected.