During a cold plasma treatment, a polymer surface is involved in many reactions, such as degradation and functionalization. CO2 plasma treatment leads to carboxylic acid function fixation onto high-donsity polvethylene surface. Attempts are done to optimize the concentration of these last functions by using CO2/H2O plasmas. By fragmentation and recombination reactions between plasma species, we hope to create new species such as carboxylic acids precursors (CO + OH or CO2 + H) that are able to increase the surface carboxylic density. Spectrochemical quantification and XPS analysis measure the surface functionalization whereas the plasma composition and plasma-polymer interactions are analyzed by mass spectrometry. The surface functionalization results show a decrease of carboxylic acid density whatever the water proportion and the discharge power explained by a mass spectrometry study. Thus, no recombination reaction leading to the formation of COOH plasma species occurred in the plasma phase, During the polyethylene treatment with CO2 plasma, evidence of degradation is also shown by mass spectroscopy ( atomic and molecular hydrogen formation in the plasma phase). A correlation done between weight-loss measurement and the atomic oxygen ((OP)-P-3 and/or (OD)-D-1) proportion in the CO2 plasma showed the significant role of the latter on the surface degradation mechanism.