Protein adsorption is an important aspect for the improvement of many applications, such as medical implants, biosensor design, etc. The density, orientation and conformation of surface-bound proteins are believed to be key factors in controlling subsequent cellular adhesion. The aim of this work is the development of a methodology in order to study in-situ and real-time protein adsorption phenomenon, and describe fibrinogen adsorption on amorphous hydrogenated carbon (a-C:H) thin films developed by rf reactive magnetron sputtering under different deposition conditions. Spectroscopic Ellipsometry (SE) in Vis-UV energy region was implemented for this purpose. SE is a non-destructive, surface sensitive technique, with the capability of performing real-time measurements in air as well as in liquid environment, with great potential in biomedical studies. An appropriate ellipsometric model has been developed, in order to describe accurately the protein adsorption mechanisms in real-time. It was found that the thickness and density of fibrinogen are larger on the a-C:H thin film deposited under absence of bias voltage application. The differences in fibrinogen thickness and transition of fibrinogen from liquid to adsorbed state are presented and discussed in the terms of the Surface and optical properties of a-C:H films. (C) 2008 Elsevier B.V. All rights reserved.