A gas sensor system based on ellipsometric readout is presented. It includes a gas chamber and a compact null ellipsometer operated in off-null mode. Small, low-cost optical components are used to demonstrate that this advanced methodology can be implemented in simplified instrumentation. The off-null ellipsometric sensing principle and transducer mechanisms of the sensing layers, as well as the instrumentation, are described. The application of the sensor system is exampled with experimental results on low-concentration alcoholic gases (methanol, ethanol, and 2-propanol) using porous silicon as a sensing layer. Optimization of the optics of the sensor system, improvement of sensitivity or alteration of selectivity by modification of sensing layers, and multisensing by using several ellipsometric units in parallel are discussed.