The synthesis and spectroscopic characterization of the chromoionophoric poly(methyl methacrylate-co-N-(9-[10-methacryloylmethyl]anthrylmethyl)monoaza-18-crown-6) (4) copolymer is described. Complexation of K+ by extraction from aqueous medium with toluene solutions of (4) was found to enhance the intensity of fluorescence up to 40% compared to the situation in which no K+ was complexed For low. concentrations. the intensin of fluorescence was found to increase linearly with the concentration of K+ in the aqueous solution The copolymer (4) might be used for the transduction of the chemical recognition of K+ into an optical signal in evanescent wave fluorescence chemical sensors based on poly(methyl methacrylate) optical waveguides.