Here we describe the chemiresistive H-2-sensing properties of drop-cast films comprised of 3.0 nm average diameter hexanethiolate-coated Pd monolayer-protected clusters (C6 Pd MPCs) bridging a pair of electrodes separated by a 23 Am gap. The gas-sensing properties were measured for 9.6-0.11% H-2 in a H-2/N-2 mixture. The sensing mechanism is based on changes in the resistance of the film upon reaction of Pd with H-2 to form PdHx, which is known to be larger in volume and more resistive than pure Pd. As-prepared Pd MPC films are highly insensitive to H-2, requiring O-3 and thermal treatment to enhance changes in film resistance in the presence of H-2. Exposure to O-3 for 15 min followed by activation in 100% H-2 leads to an increase in film conductivity in the presence of H-2, with a detection limit of 0.11% H-2. When exposed to temperatures of 180-200 degrees C, the conductivity of the film increases and a decrease in conductivity occurs in the presence of H-2 with a detection limit of 0.21%. The sensing behavior reverses after further heating to 260 degrees C, exhibiting an increase in conductivity in the presence of H-2 as in O-3-treated films and a detection limit of 0.11%. The sensitivity of the variously treated films follows the order O-3 > high temp > low temp, and the response times at 1.0% H-2 range from 10 to 50s, depending on the treatment. FTIR spectroscopy, Raman spectroscopy, and atomic force microscopy provide information about the C-6 monolayer, Pd metal, and film morphology, respectively, as a function of O-3 and heat treatment to aid in understanding the observed sensing behavior. This work demonstrates a simple chemical approach toward fabricating a fast, reversible sensor capable of detecting low concentrations of H-2.