Abstract: An apparatus for coating surfaces of objects by a gas deposition method under vacuum conditions in process chambers, including at least one treatment chamber for receiving the objects to be coated and at least one additional process chamber, connected to the at least one treatment chamber, for conducting gases to be deposited and/or discharging portions of non-deposited partial gas quantities. A measuring unit for detecting the coating thickness on a surface or sections of the surface of the process chambers is arranged in at least one additional process chamber. The measuring unit is connectable to a control and evaluation unit. The measuring unit detects ACTUAL data of the coating thickness on a surface of the at least one additional process chamber as measured value and these ACTUAL data are forwarded to an evaluation device, in which these ACTUAL data are compared with NOMINAL data.