The disclosure relates to a thermometer having an exchangeable measurement insert for measuring process temperatures of liquid or gaseous media in vessels of process installations, and to a method for replacing the measurement insert. On the basis of a thermometer in which the measurement insert is accommodated, such that it can be changed, in a protective tube which is closed at one end and has a process flange, the closed end of the protective tube being immersed, as far as the process flange, in a container in which the medium whose temperature is to be determined is situated, and the open end of the protective tube being guided at least indirectly to a connection housing in which at least connection terminals for connecting the measurement insert to a remote measured value processing system are arranged, provision is made for the transition from the open end of the protective tube to the connection housing to have a clearance fit with a cylindrical fitting surface between the measurement insert and a fitting sleeve which at least partially surrounds the latter.