The absorption and fluorescence of free base, tetraphenyl-substituted porphyrins are shown to be sensitive to exposure to NO2. The Soret band absorption red-shifts and fluorescence is quenched by an order of magnitude. The fluorescence spectrum of a gassed sample is also red-shifted and broadened following exposure. Several solid state embodiments of a fluorescent sensor based upon free base porphyrins are proposed.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.