Ellipsometry has sufficient sensitivity for sensor applications and is here used as an optical readout method in a gas sensing system. Porous silicon is used as sensing layers in which vapors of solvents can adsorb and condensate due to capillary effects. A miniaturized multi-beam ellipsometer system is proposed and the concept is demonstrated by measurements on alcohol vapors. Optimization of the sensor system is discussed and improvement of sensitivity and alteration of selectivity by metal deposition in porous silicon layers are presented.