This paper investigates the feasibility of a micro-optical sensor for species concentration. A theoretical analysis was carried out to model the sensor performance followed by a series of experiments. The measurement concept is based on optical mode shifts of dielectric spheres with diameters of the order of 100 μm. The optical modes, which are also called the "whispering gallery modes" (WGM), are excited by coupling light into the sphere using an optical fiber. A change in the index of refraction of the medium surrounding the sphere induces a shift in the optical modes of the sphere. Therefore, by monitoring the WGM shifts, one can determine the concentration of a given species in the medium surrounding the sphere. In the theoretical analysis, we used a simple, heuristic approach to model the WGMs in the sphere that is based on geometric optics and the basic electromagnetic equations of wave reflection. The resulting equations describe the dependence of WGMs on the refraction index of the surrounding medium. These equations indicate that the WGMs are not sensitive enough to index of refraction changes in the case of gas media. However, they can be sufficiently sensitive for the development of micro-sensors in liquid media. Experiments were carried out to validate the theoretical analysis and to provide a first order assessment of this sensor concept. The concentration of two types of salts, potassium carbonate and sodium bicarbonate were measured in water.