Abstract: A fast-response colorimetric carbon dioxide indicator device which is substantially insensitive to humidity and comprises a chemically inert substrate and an indicator composition supported by the substrate and responsive to exposure to carbon dioxide in expired respiratory air to undergo a color changing reaction. The indicator composition includes a pH sensitive dye and a basic substance having the general formula in which X is a nitrogen or phosphorus atom, each of R1, R2, R3 and R4 is an alkyl, Y− is an anion selected from the group consisting of hydroxide, fluoride, chloride, bromide, iodide, carbonate, and tetrafluoroborate, at least one of the alkyls R1, R2, R3 and R4 having at least 13 carbon atoms and at least one of the other alkyls having from 6 to 8 carbon atoms, the remaining alkyls, if any, having from 1 to 12 carbon atoms.