Supported palladium particles are used as catalysts in many processes. In the course of investigation of Pd-based catalysts used for the selective hydrogenation of nitro-arenes to amino-arenes, in addition to the presence of the modes of the (expected) hydrogen-in-metal system β-PdH, we observed an additional mode at ~2100 cm-1. We assign the mode to hydrogen chemisorbed in the on-top position i.e. directly above a surface Pd atom. This site has never before been observed on palladium and periodic-DFT calculations support our assignment. palladium hydrogenation catalysts have been extensively studied for more than 50 years, so the detection of a new catalytic site is very significant. In view of the potential importance of the on-top Pd¿H we propose to use the method developed previously to make a structural determination of the on-top site of hydrogen on palladium.