Abstract: The quality of a weld is usually monitored via the different wavelength radiation issuing from the weld point. During the welding of workpieces, however, this radiation (S) is also modulated by the instantaneous width (b) of the gap existing between the workpieces. To avoid the generation of unnecessary fault signals (F) on the one hand and excessive tolerance bands for the permitted signal range on the other hand, the invention proposes that a correction formula (.increment.S(B)) which reproduces the dependency of the radiation signals (S) on the gap width (B) be produced in a preliminary test. The gap width (B) in the weld point is defined during welding. A correction value (.increment.S(B)) is defined during the welding process on the basis of the correction formula (.increment.S(B)) and the instantaneous gap width (B) in the weld point and the limit values (MAx(B.sub.0), MIN(B.sub.0) previously defined for a gap width zero are displaced by this correction value (.increment.S(B)).