Selective detection of trace level water vapours by functionalized porous silicon (PS) using photoluminescence (PL) quenching technique is reported. This sensitive technique provides sensing data through optical probing on a surface with a submicron surface area. PS samples were prepared via electrochemical etching and were functionalized using ammonium sulphide and UV exposure which brought about major changes in the surface properties. Samples were examined using SEM, FTIR, contact angle and PL spectroscopy. These samples were tested as sensors in the presence of linear aliphatic alcohols and water vapours in the wide range of 0-400 ppm. An increasing trend of sensor response with increase in alcohol chain length and minimal response to water vapours was observed from sensors based on as-anodized PS samples whereas post functionalization, sensors portrayed an opposite response wherein selective response to water vapours and minimal response to alcohols was indicated. The role of surface nitridation is highlighted in selective sensing of water vapours. A correlation between the sensing response and surface chemistry of samples has been established. It is implicated that sensing is directly linked to both the chemical properties of the sensing surface and analytes. A highly sensitive sensor for detection of water vapours at low ppm is presented. A simple and compact design of portable optical sensors, applicable for water vapour sensing is proposed. © 2016 The Royal Society of Chemistry.