A gas concentration sensor comprising a long period grating with a high refractive index coating is reported. The deposition of a high refractive index coating on the optical fiber allows a reorganization of the cladding modes which leads to a substantial increase in the sensitivity to the surrounding refractive index. To maximize the grating sensitivity in the vicinity of the refractive index of CO 2 , the grating was coated with a 365nm layer of polystyrene. The results show that the sensor is able to distinguish between the CO 2 and N 2 gases with high reproducibility and reversibility. The sensitivity of the sensor is 1.23pm/%CO 2 .