A method for commissioning and personalizing a subscriber identity module in a mobile communications network, in which the subscriber identity module, prior to its first-time commissioning in the mobile communications network, does not yet contain an individual, unique parameter data set, comprising at least an individual subscriber identification (IMSI) and an individual secret key (Ki), and is equipped with an individual, unique parameter data set only after or on the occasion of its first-time commissioning in the mobile communications network, characterized by the steps of: a) supplying a subscriber identity module which contains a provisional parameter data set which comprises: a1) a unique subscriber identification (IMSI) individual to the subscriber identity module, and a2) a non-individual, non-unique provisional secret key (Ki-G) identical for a plurality of subscriber identity modules; b) supplying a mobile communications server which has the non-individual, non-unique provisional secret key (Ki-G) and, at the mobile communications server, performing the steps of: b1) accepting from the subscriber identity module an authentication message (N) formed with the individual, unique subscriber identification (IMSI) and the non-individual, non-unique provisional secret key (Ki-G); b2) in response to the acceptance of the authentication message (N), verifying the authentication message (N) in accordance with predetermined criteria, and, in the event of successfully verifying the authentication message (N), sending an individual, unique final secret key (Ki) to the subscriber identity module for programming into the subscriber identity module. a) supplying a subscriber identity module which contains a provisional parameter data set which comprises: a1) a unique subscriber identification (IMSI) individual to the subscriber identity module, and a2) a non-individual, non-unique provisional secret key (Ki-G) identical for a plurality of subscriber identity modules; b) supplying a mobile communications server which has the non-individual, non-unique provisional secret key (Ki-G) and, at the mobile communications server, performing the steps of: b1) accepting from the subscriber identity module an authentication message (N) formed with the individual, unique subscriber identification (IMSI) and the non-individual, non-unique provisional secret key (Ki-G); b2) in response to the acceptance of the authentication message (N), verifying the authentication message (N) in accordance with predetermined criteria, and, in the event of successfully verifying the authentication message (N), sending an individual, unique final secret key (Ki) to the subscriber identity module for programming into the subscriber identity module. a1) a unique subscriber identification (IMSI) individual to the subscriber identity module, and a2) a non-individual, non-unique provisional secret key (Ki-G) identical for a plurality of subscriber identity modules; b) supplying a mobile communications server which has the non-individual, non-unique provisional secret key (Ki-G) and, at the mobile communications server, performing the steps of: b1) accepting from the subscriber identity module an authentication message (N) formed with the individual, unique subscriber identification (IMSI) and the non-individual, non-unique provisional secret key (Ki-G); b2) in response to the acceptance of the authentication message (N), verifying the authentication message (N) in accordance with predetermined criteria, and, in the event of successfully verifying the authentication message (N), sending an individual, unique final secret key (Ki) to the subscriber identity module for programming into the subscriber identity module.