We synthesized 3,3-dimethyl-1'-octadecyl-6-nitrospiro-[2H-1-benzopyran-2,2' indolinel] (SP-18) and examined its photochromic behavior and fluorescence in the presence of acid. When strong acids such as HCl and dodecylbenzenesulfonic acid (DBSA) were present, the merocyanine (MC) form generated via heterolytic cleavage of oxygen-carbon bond of the spiropyran (SP) by the irradiation of UV light was protonated to produce the complex MCH+. The MCH+ had a yellow color with the absorption maxima at between 410 similar to 430 nm. Since ring-closing reaction of the MC form to produce the SP form was retarded by the protonation of the phenolate in the MC form, MCH+ had better stability than the MC. Emission peak of the MCH+ form was observed at 645 nm with HCl and 600 nm with DBSA, respectively. Polymethylmethacrylate (PMMA) films containing the MCH+ retained about 75% of its original absorbance even after 48 h.