Injection-moulded plates of four commercial thermoplastic polyolefins (TPOs) were subjected to oxygen plasma treatment. The modified surfaces were analyzed by contact angle measurements and x-ray photoelectron spectroscopy (XPS), and the adhesion properties of the plates were evaluated by a 90° peel test after being lacquered with a two-component polyurethane lacquer. The study included treatments in two different plasma rectors operating at different frequencis. The influence of certain process parameters, such as discharge power, flow rate and gas pressure, was investigated, as was that of frequency (using the same reactor). While the results revealed that oxugen plasma treatment indeed led to improved wettability, the degree of surface modification was not highly affected by changes in the processing conditons. In contrast, there was a great effect on the lacquer adhesion , in particular by changes in discharge power and gas pressure. The results also showed that the TPOs were sensitive in different ways towards changes in the processing conditions. Two of the TPOs revealed good lacquer adhesion for a wide range of processing conditions, while the others were more sensitive. It was also found that, regardless of the absoloute peel force, the failures occured in the substrate at some distance below the oxidized layer. These observations were attributed to a VUV-induced formation of radicals which, in the case of polypropylene-based materials, predominantly lead to B-scissions. As secondary radicals have a higher tendency to form crosslinks that can compensate for chain scission reactions, the difference in the sensitivity of the TPOs was proposed to be related to the amount and distribution of ethylene in the materials.