In previous works it was demonstrated that the electrical resistivity of Polypyrrole (PPY) changes when exposed to different organic solvents which allowed the development of applications in gas sensors [1,2]. Also, is well known that optical gas sensors have several advantages over conventional electronic ones like high sensitivity, reduced signal-to-noise ratio, and compatibility with combustible gases. The optical properties of polymer materials have became of great importance in modern optical design of polymer based optical sensors and devices. Thin polymer films appear in an ample spectrum of applications such as photonics, data storage, communications and sensor devices [3]. In this work an optical sensor for the detection of water vapor using Polypyrrole (PPY) as active material is proposed. As a first step in studying polypyrrole for this application, the refractive index of this material was measured after the films were exposed to water vapor, and the results showed a variation of the refractive indices of the polymer in the wavelength of 632.8 nm. Finally, an optical device was fabricated using integrated optics technology over silicon, which uses polypyrrole as active layer for sensing. The results of the characterization of this optical device showed that for relative humidity concentrations above a specific value (∼70%) the optical power at the output of the device decays to insignificant values, which allows for the device to be used as an optical switch. © 2012 SPIE.