Unsupported, bulk phase palladium sulfide has been studied for the selective hydrogenation of acetylene. The sample underwent significant change during thermal pretreatments, the extent of which depends on temperature. Exposure to hydrogen at temperatures of 150 degrees C or above results in the loss of sulfur from the sample, primarily as hydrogen sulfide. As sulfur is lost, the sample is progressively transformed from a sulfur rich phase (PdS) to a sulfur lean phase (Pd4S) via an intermediate phase (Pd16S7). Reduction at 250 degrees C produces a material, which contains Pd4S as the surface phase, whereas reduction at 350 degrees C results in a largely pure Pd4S phase. Thermal treatments which produce a Pd4S surface display excellent catalytic properties. At complete acetylene conversion at 250 degrees C, ethylene selectivities of 82.8% and 90.0% were obtained under non-competitive and competitive conditions, respectively. Beneficial catalytic properties arise from the uniformity of Pd sites due to the crystal structure of Pd4S along with electronic influences on the adsorption/desorption processes arising from sulfur neighbours. No indications of deactivation or significant deposition of carbon were observed over Pd4S sample after 50 h on stream. (C) 2018 Elsevier Inc. All rights reserved.