An optical sensor for aqueous l-butylamine is presented which combines two novel techniques: A fluorescent indicator dye (fluoro reactand) embedded in a thin polymer layer performs a reversible chemical reaction with the analyte, causing changes in luminescence intensity. At the same time, inert phosphorescent beads dispersed within the polymer layer provide luminescence signals that act as an internal reference for the indicator dye. As a consequence, the optical sensor is independent of light source fluctuations, ambient light, drifts in optoelectronic setup, or optical fiber bending.