The forward and rear calorimeter of the ZEUS experiment consists of 48 modules. Before their installation into the ZEUS detector they were calibrated at DESY using cosmic ray muons in order to check their performance and to compare the response to cosmic muons to the response, obtained for some modules, to 100 GeV beam muons. The setup, the test procedure and the analysis of the data are described in this paper. The relative calibration of the different modules, as well as of the different cells within a module can be obtained with cosmic ray muons with an accuracy of about one percent for a measurement time of 3–5 days/module.
