MoS2, in single to few-layer format, is of interest because of its potential for advanced transistor and sensor applications. Its sizable bandgap enables single layer transistors with large on/off current ratios, and the large surface-to-volume ratio provides sensitive transduction of surface physisorption to the channel conductivity. Here, we discuss aspects of transistor device fabrication and of chemical vapor sensing experiments. We expose MoS2 chemical sensors to a variety of analytes, find the largest response to triethylamine, a nerve gas by-product, and explain our results based on a donor-acceptor model. We show that our MoS2 sensors provide comparable sensitivity and much higher selectivity than other low-dimensional sensors such as carbon nanotube and graphene chemical sensors. We present results for back-gated sensing and light sensitivity for our monolayer MoS2 sensors, and compare the results with multilayer MoS2 sensors. Published by Elsevier Ltd.