The concept of an ellipsometric sensor array system - "an optical nose" - is presented. The system is used to monitor and discriminate gases. The optical readout is based on polarizer-compensator-sample-analyzer (PCSA) off-null ellipsometry. Thin porous silicon sensing layers with and without surface chemical modifications are used to measure low concentration methanol, ethanol and 2-propanol gases. The main contents of the thesis are:

- Principle and construction of a prototype PCSA ellipsometric gas sensor system.

- Optimization of the azimuth angle settings of the optical components in PCSA off-null ellipsometry to obtain the maximum readout sensitivity.

- Application of return-path off-null ellipsometry in gas sensing as an alternative way to construct ellipsometric gas sensors.

- Modification of porous silicon sensing layers by copper deposition to obtain different gas sensitivity for the enhancement of selectivity.

- Multi-sensing concept of the optical nose to detect and discriminate alcohols using porous silicon sensing layers and pattern recognition techniques.

So far, a prototype PCSA ellipsometric gas sensor system has been constructed and tested. Low concentration methanol, ethanol and 2-propanol vapors are detected and discriminated based on the PCSA off-null ellipsometric gas sensor system and porous silicon sensing layers.