The optical properties of electrochemically polymerized N-methylaniline are changed in the presence of gaseous hydrogen chloride. This effect was used for preparation of chemical sensors with transduction based on surface plasmon resonance. The interaction of hydrogen chloride with a two nm layer of poly-(N- methyl aniline) on gold leads to the shift of the surface plasmon resonance. The analysis of the resonance spectra demonstrates that the effect is caused by an increase of the imaginary component of the refractive index; a minor contribution is also provided by film condensation leading to an increase of the real component of the refractive index and a decrease in the thickness. The effect is selective and quasi-reversible. The concentration dependence of the gas effect obeys Langmuir ' s adsorption isotherm with a reciprocal value of the binding constant of 850 +/- 160 ppm. (c) 2004 Elsevier E.V. All rights reserved.