Abstract: An optical sensor for detection of gases such as nitrogen oxides includes a thin film of an active material comprising a highly polarisable organic material deposited by the Langmuir-Blodgett technique or otherwise on a substrate and interrogated optically, to detect changes in at least one optical property such as reflectance or complex refractive index. Preferred materials include a specific polysiloxane and low-molecular-mass compounds (FIGS 1 to 5b), having specific electron donating and/or accepting groups, including azobenzenes. The optical change may include a visible colour change.