A micromachined silicon Fabry-Perot interferometric sensor is demonstrated as an optical chemical sensor. This sensor is based on the combined nature of the amplifying and tuning characteristics of the Fabry-Perot microcavity structure and the doping effect of polymer films such as Poly(3- dodecylthiophene) (P3DDT) upon exposure to an oxidizer, in this case, iodine. The fabricated Fabry-Perot chemical sensors show reversible sensing behavior with a maximum change in transmitted optical intensity of 60%. Significant improvement of the sensing performance is obtained from the Fabry-Perot microcavity structure compared to a simple planar single membrane structure, which indicates the resonant effect of the Fabry-Perot cavity on the chemical sensor. The measured sensing characteristics suggest that the change in absorptance of P3DDT polymer inside the microcavity plays a major role, while the deflection of a microcavity membrane by the P3DDT polymer-induced surface tension gives tunability of the sensor to maximize the amplification of output response by adjusting the Fabry- Perot microcavity gap spacing.