A gas sensing device based on hydrocarbons (CHx)/porous silicon structure was fabricated. Porous silicon was prepared by electrochemical etching of p-type silicon in HF/ethanol (50/50) by volume. The porous samples were coated with hydrocarbons groups deposited by plasma of methane. Sensitivity of these devices, response time and capacitance response to different gas exposure (ethane, ethylene and propane) have been investigated. Current-voltage and capacitance-voltage characterizations show that heterojunction are modified by the gas reactivity on the PS/CHx surface. Based on these results and IR spectroscopy measurements, mechanism of electrical conduction was interpreted in term of charge carrier transport through shallow traps associated with the surface. Finally, it was noted that, after 6 months, our sensor preserves its features such as, short response time, stability and reproducibility. (C) 2004 Elsevier B.V. All rights reserved.