A sulfonephthalein dye was immobilized in sol-gel-derived thin films fabricated from mixtures of tetraethoxysilane and methyltriethoxysilane. Evanescent wave absorption spectra on optical fibres were monitored on exposure to gaseous ammonia. Analysis reveals nonclassical response behaviour, the extent of which is related to the polarity of the glass. This is accounted for in terms of dye solvation and the involvement of the micro-heterogeneous glass surface in the indicator reaction. These observations place important restrictions of the suitability of polar dye/sol-gel-derived glass combinations for gas phase analysis.