An optimization strategy for a generic absorption-based optical chemical sensor that employs a single-reflection planar configuration is reported. A theoretical model describing the sensor sensitivity is presented and verified experimentally. It is shown that optimum sensitivity is not achieved with an evanescent-wave sensing technique but with a configuration in which the interrogating light propagates within the sensing layer. Moreover, an optimization strategy based on identification of an optimized reflection angle is described. This analysis provides an optimization strategy that is extendable to multimode waveguide platforms. The predictions of the model are used in the design of a prototype LED-based sensor system. The performance of this system is examined, and the results are compared with alternative absorption-based sensor configurations.