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Abstract: Two indicator dyes are presented that perform reversible chemical reactions with neutral analytes in thin layers of plasticised poly vinyl chloride (PVC). Since these dyes combine the properties of a selective ligand with those of a chemical reagent, they are termed chromoreactands. The lipophilic decyl derivative of alpha-Naphthyl Red (CR-418) reversibly reacts with formaldehyde shifting the absorbance maximum in plasticised PVC from 438 to 449 nm. The sensor membrane exhibits highest sensitivity to formaldehyde in the 1-100 mM range with a detection limit of 0.3 mM. A bisazo dye with a reactive trifluoroacetyl group (CR-573) reversibly forms a hemiaminal with amphetamine in thin layers of plasticised PVC and changes its colour from blue to red (corresponding to a shift in absorbance maximum from 575 to 548 nm). The sensitivity of the dye in the polymer layer covers the range from 0.3 to 30 mM amphetamine with a detection limit of 0.1 mM. The same dye also detects ethanol when the catalyst tridodecylmethylammonium chloride is used to accelerate the chemical reaction. The sensor layer exhibits a sensitive range from 1 to 40% ( v/v) ethanol and a detection limit of 0.8% (v/v). Maximum signal changes are observed at 630 mn making CR- 573 compatible with cheap light sources and detectors. (C) 2003 Elsevier Science B.V. All rights reserved