We have fabricated, and characterized a Bragg grating, having a pitch of 10 μm, directly etched on a porous silicon-based planar waveguide to be used as an optical sensor for chemical substances. The transmission spectrum of this device shows a well-defined peak in the near infrared wavelength region due to a high order resonance of the Bragg grating. On exposure to different volatile organic compounds the peak undergoes distinct shifts. The phenomenon is fast and completely reversible. Limits of detection of hundred of ppm can be estimated from the experimental data.