The method and device are used to plasma-treat workpieces. The workpiece is inserted into a chamber of a treatment station that can be at least partially evacuated. The plasma chamber is bounded by a chamber bottom, a chamber cover, and a lateral chamber wall. The method process is optically monitored at least at times. In the optical monitoring, spectral lines of the radiation of the plasma above 500 nanometers are evaluated. Preferably, the evaluation is performed for frequencies above 700 nanometers.