In order to detect the exchange of a module, identified by a serial number, in a microprocessor system,a code number, which is obtained from the serial number by using an encryption method, as well as information required for calculating the serial number from the code number, are stored in the microprocessor system;the code number is read and an unencrypted serial number is calculated from the code number with the aid of the information; andthe decrypted serial number thus obtained is compared to the serial number of the module and the module is detected as exchanged if its serial number does not match the decrypted serial number.