A clad-modified fiber optic sensor with nanocrystalline CeO2 is proposed for gas detection. As-prepared and annealed CeO2 (500 C) samples have been used as gas sensing media. The spectral characteristics of the fiber optic gas sensor are studied for various concentrations of ammonia, ethanol and methanol gases (0-500 ppm). The sensor exhibits linear variation in the spectral peak intensity with the gas concentration. The characteristics of the sensor are also studied for gas selectivity. The time response characteristics of the sensor are reported. © 2013 Elsevier B.V. All rights reserved.