Two new hemicyanine dyes with dihydroxybenzene moiety as a donor group have been investigated in terms of interacting with volatile organic compounds (VOCs) EtNH2. The sensing behavior of the hemicyanines toward EtNH2 was studied by UV–vis absorption spectroscopy. Different sensing mechanism of the dyes to EtNH2 was confirmed by 1NMR studies, together with theoretical calculations based on DFT and PPP-MO methods. We have also investigated the fast EtNH2 gas sensing of these dyes loaded poly(acrylonitrile) nanofiber. Reversible response and recovery were achieved using alternating gas exposure. This system shows a fast EtNH2 gas sensing within 5 s.