By using ellipsometry at the air-water interface, we determined the optical parameters of a mixed monolayer containing an anionic phospholipid matrix (DMPA), and a cationic porphyrin (Ni-TMPyP, Ni(II)-tetrakis(4-methylpyridyl) porphyrin) as large counter ion, in molar ratio Ni-TMPyP/DMPA= 1:4 to further describe the organization of this film. The nulling ellipsometric measurements were done on two phases observed directly by Brewster angle microscopy, over specific regions with a size of few microns. Therefore, values of the ellipsometric angles for the different regions at the interface (domains and surrounding areas) have been obtained. The results have been interpreted applying the "one-layer" optical model being in good agreement with the predictions from the geometrical models. Further, the strong increase of the Delta ellipsometric angle with respect to the DMPA film has been interpreted as due, mostly, to the partial dehydration of the polar group whatever increase of the refractive index of the film. (c) 2005 Elsevier B.V. All rights reserved.