Optical characterization of thin porous platinum films has been carried out by spectroscopic ellipsometry in air, in water and in methanol. From the measurements in air, the thicknesses and porosity of the films were determined. The measurements in water showed that air pockets are present at the interface between water and a platinum film if the sample surface is made hydrophobic by silanization. In methanol such platinum films are wetting and the air pockets vanish. In the optical analysis it was necessary to include a thin organic layer on the hydrophobic surfaces. However, no organic layer or air pockets were found in water on as-prepared hydrophillic surfaces. Furthermore, upon storage in laboratory air, an organic layer formed spontaneously on the hydrophillic surface. The ellipsometric results support the suggestion that, by means of the air pockets, hydrophobic thin platinum films act as gas permeable structures in electrolytes which leads to a possibility to develop chemical sensors for molecular species in solutions.