An innocuous fluorescent species, pyrene benzothiazole, PyB, has been found to exhibit outstanding colorimetric and fluorogenic properties toward the estimation of toxic phosgene gas via significant participation of two pivotal photo-physical processes, photoinduced electron transfer (PET) and intramolecular charge transfer (ICT), as manifested through occupation of the lone electron pair on the benzothiazole N atom upon quaternization by phosgene. This is the first report of a phosgene sensor that harnesses most efficiently the charge transfer potential inherent in the sensing platform. In addition, since the basicity of the benzothiazole N atom is not quite sufficient to warrant easy interaction with less acidic chlorides and/or acids than the severely potent phosgene, the probe shows a high degree of selectivity. PyB is amenable to nanomolar (nM) level phosgene sensing through naked eye observation of colour changes in plain daylight as well as the presence of a handheld UV lamp with an appreciably low response time of <50 seconds.