The gas sensing characteristics of nanocrystalline samarium oxide (Sm2O3) clad modified fiber optic gas sensor are reported with and without a carrier gas (air) in the radiation mode (detection of a light radiating from the clad modified surface) and transmission mode (detection of a light transmitted through the fiber) for ammonia, ethanol, methanol and acetone gases. When the carrier gas present, the sensor output in the radiation mode increases for methanol and ammonia whereas it decreases for ethanol and acetone with the increase in the gas concentration. It is reverse, in the case of, the transmission mode. When there is no carrier gas, the sensor output decreases in the radiation as well as transmission modes for all gases. The dynamic characteristics of the gas sensor are presented. © 2018 Elsevier Inc.