Abstract The catalytic deactivation of R-NiSn catalysts was explored during hydrogen production by aqueous-phase reforming (APR) of ethylene glycol. X-ray photoelectron spectroscopy and CO and H 2 adsorption microcalorimetry were combined with previous characterization to show that R-NiSn catalysts are composed of a Ni core surrounded by a Ni 3Sn alloy after heat treatments above 533 K. Adsorption studies (CO, H 2, and N 2), XPS, X-ray diffraction, and thermogravimetric analysis show that R-Ni 15Sn catalysts deactivate by interaction with water under APR reaction conditions, rather than coking or Ni(CO) 4 formation. Over the first 48 h on stream, deactivation proceeds rapidly by the sintering of small Ni particles and by the formation of NiSn surface alloys with lower catalytic activity and higher selectivity for the production of hydrogen by APR of dilute feed solutions. After several days on stream, R-Ni 15Sn catalysts deactivate at a slower rate because of oxidation and dissolution by water, leading to Ni effluent concentrations near 50 wppm after 240 h on stream. The first-order deactivation constant can be improved from k d = 0.0020 h −1 to less than 0.0001 h −1 between 140 and 240 h on stream with the use of rigorous heat treatments at 623 K in H 2 to form resilient NiSn alloys prior to reaction and/or with the use of energy-efficient stoichiometric feeds (water/ethylene glycol = 2).