A new type of optical chemical sensor recently developed in our lab has been demonstrated for highly sensitive, in-situ detection of explosives. The sensor is comprised of a dense silica thin film grown on the straight-cut endface of a standard, 125μm telecommunication optical fiber. Silicalite is an all-silica MFI-type zeolite with an effective pore size of 0.55nm. MFI zeolite is highly hydrophobic and selectively adsorbs organics of appropriate molecular size. The sensor device operates through measuring the optical refractive index or optical thickness of the coated zeolite film which changes in response to the adsorption of molecular species in its crystalline structure. In this work, the sensor exhibited different responses to simulants including pxylene, o-xylene, and triisopropylbenzene and trinitrotoluene (TNT) trace vapor in helium carrier gas.