There is provided a composition for the detection of an electrophilic gas, such as chlorodifluoromethane or carbon dioxide, which comprises a Lewis base capable of removing a proton from the gas or reacting therewith in a similar electrophilic manner; a dye capable of visibly indicating a color change on protonation or deprotonation; a solvent for the dye, the base and the gas; and a rheology modifier capable of producing a non-newtonian gel of all of these components which is sufficiently translucent to permit visual detection of change of color of the dye and of sufficient pseudoplasticity/thixotropy to provide adhesion to vertical and horizontal surfaces.