Cesium-promoted ruthenium nanoparticles supported on multi-walled carbon nanotubes catalysts are shown to be highly active for hydrogen production by ammonia decomposition. Its low temperature activity is significantly improved as the cesium loading increases, reducing the activation energy from 96.7 kJ/mol in the absence of cesium to 59.3 kJ/mol with a cesium/ruthenium molar ratio of 3. Hydrogen production was observed to proceed below 590 K which represents a breakthrough towards the use of ammonia as chemical storage for in-situ hydrogen production on fuel cells. The catalytic enhancement is shown to be due to the electronic modification of ruthenium by the electron donating cesium promoter located on the ruthenium surface and in close proximity on the CNT surface. However, higher promoter loadings above a cesium/ruthenium ratio of 3 leads to ammonia inaccessibility to the catalytic active sites. Copyright (C) 2014, The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications, LLC.