Thin films were deposited from a mixture of ethylene and acrylic acid in RF (13.56 MHz, parallel plate) glow discharges in different experimental conditions to obtain substrate-adherent coatings stable in water, surface-functionalized with tunable density of carboxyl groups. The surface composition of the coatings as well as their stability in water was investigated by X-ray Photoelectron Spectroscopy measurements and Toluidine Blue-O derivatization of the carboxylic acid functionalities. Coatings stable in water media were obtained, with tunable surface density of -COOH groups, of possible interest in biomedical applications. Such coatings were used in cell culture experiments with Saos-2 osteoblast cell lines.