This thesis describes the development of a dual-beam, fiber optic instrument for solution-phase analysis of thin-film sensors. Colorimetric indicators are described for incorporation into porous polymeric films. The films are immobilized at the distal ends of optical fibers and yield an optical response proportional to the target analyte. In its present form, the instrument functions in an absorbance mode as a solid-state, dual-wavelength photometer and is equipped with a custom-designed microprocessor controller and data processor. The described study encompasses interdisciplinary research in the fields of biomedical engineering, synthetic organic chemistry, interfacial chemistry, materials science, and computer and electrical engineering;The primary emphasis in development of thin-film sensors focused on pH. Other indicators are proposed for detecting calcium, potassium, and sodium. These ions are major physiological indicators of homeostasis; therefore, monitoring of these parameters is important in determining the clinical status of astronauts on long-term space missions.