An azo-dye-incorporated polyionene was spin-coated as thin films on silver evaporated glass substrates. The films were used in conjunction with surface plasmon resonance (SPR) spectroscopy to examine the utility of combining these techniques for optical H+-ion sensing. A method suggested by Kano and Kawata was used to enhance the sensitivity of the classical SPR measurements toward absorption detection. With the exposure of the composite polyionene films to HCl vapor, changes in the reflectivity signal were obtained in proportion to the concentration of H+ ions and according to the analyzing wavelength. The transformation of the parent azobenzene into the azo-hydrazone polar form determined the displacement of the absorption band and was the basis for the absorption-enhanced SPR measurements. The difference in the SPR signals for the neutral sample and that exposed to interaction with HCl was most demonstrated for the wavelength at which the sample spectrum changed from being nonabsorbing to absorbing. The results show that chromophoric polyionenes are good materials for the production of sensitive substrates and can be combined with SPR spectroscopy for the optical sensing of aggressive gaseous species.