A microwave plasma treatment in a down stream configuration was used to modify the natural hydrophobocity of untreated wool fibers. This property is a consequence of the presence of a Fatty acid monolayer (F-layer) on the outermost part of the fiber surface. The wool fibers treated with plasma were analyzed by means of X-ray photoelectron spectroscopy (XPS) without previous exposure to the air. Experiments have been carried out with air, water vapor, oxygen and nitrogen as plasma gas. The "in situ" analysis of the treated samples has permitted to differentiate between the plasma effects and those other linked to the exposure of the fibers to the air after their treatment. The results have evidenced the effects induced by the different active species generated by plasma from the different components of the air. In general, the intensity of C-C peaks decreases and that of the C-O, C=O and O-C=O increases when using a gas containing oxygen species. Simultaneously, the intensity of the S-S groups decreases and that of the sulphonate (SO3-) increases. Other changes are also detected in the intensity of the N Is level. The extent and characteristics of the oxidation and functionalisation of the hydrocarbon chains of the F-layer depend on the nature of gas. Thus, whereas treatments with plasmas of air and water vapor strongly affect the hydrocarbon chains of the F-layer, oxygen is less effective in the oxidation process. It has been also noted that the active species formed in the nitrogen plasma do not induce any significant change in the surface composition of the wool fibers. (c) 2005 Elsevier B.V. All rights reserved.