The development of fibre optic ammonia and pH sensors using analyte-sensitive reagents entrapped in a sol-gel-derived silica film is presented. These sensors are based on evanescent wave absorption using unclad fibres dipcoated with the chemically-sensitive coating. A detailed characterisation of film properties was carried out. Film properties were monitored as a function of various processing parameters, and the results are interpreted in terms of the chemical reactions involved in the sol-gel process. A pH sensor was constructed using sulfonephthalein dyes immobilised in sol-gel-derived coalings. Its range was extended by co-immobilisation of two indicators. A detailed study of the parameters affecting the leaching of reagents from sol-gel films is presented. Ammonia sensing using pH and ammonia-specific reagents, immobilised in sol-gel silica and polymer coatings is detailed. The issues involved in choosing a pH indicator for ammonia sensing is discussed. Issues affecting the response time and sensitivity of these ammonia sensors are highlighted. These include the choice of sol-gel film, dye concentration and humidity dependence. Ammonia sensing using novel near-infrared absorbing reagents is discussed. In addition the issues affecting the diffusion of metal ions into sol-gel films is discussed. The reaction of xylenol orange with Pb; ' in solution is used as an example. A number of characterisation systems have been employed, and these are described in detail.