A new approach for developing absorbance optochemical sensors is presented in this paper. It is based on a planar microoptic circuit where an optochemically active membrane, which responds selectively to a compound, is deposited in the device, yielding a part of the guiding planar structure. Light is propagated through the membrane, which changes its spectral absorption properties and controls the selectivity of the measurements by means of several immobilised compounds. This way, high sensitivity of the device can be easily obtained due to relatively long light paths through the membrane, and low response times can be achieved as the analyte diffusion occurs perpendicularly to the light path and through a thin membrane. Experimental results on measurements of the concentration of a specific ion in solution using the fabricated sensors are also presented.