Polypyrrole (PPy) was modified with ferrocene (Fc) by direct incorporation during polymerization so as to increase its sensitivity for carbon monoxide (CO) gas sensor applications. Synthesis was carried out by chemical oxidative polymerization in the presence of ferrocene using ferric chloride. The presence of these species in PPy was confirmed by different characterization techniques. The CO response measurements were carried out for the sensors fabricated in a surface cell mode with active PPy + Fc films by exposing to CO gas (300 ppm) at room temperature and it was found that the response as well as the recovery characteristics of these materials is very fast and dependent on the film composition. The response factor was found to be maximum for a 1.32 mol% Fc content in the polymer. These various results can be explained on the basis of interaction of CO molecules with Fe atoms of the ferrocene together with charge transfer interaction with the PPy chain. © 2007 Elsevier B.V. All rights reserved.