A　series　of　Ru/Al2O3　catalysts　were　prepared　to　study　the　effect　of　the　amount　and　the　origin　of　residual　chlorine　on　chemisorptive　property　and　the　ammonia　synthesis　activity.　The　catalysts　were　characterized　by　X-ray　fluorescence,　CO　chemisorption,　transmission　Electron　Microscopy,　X-ray　photoelectron　spectroscopy,　hydrogen　temperature-programmed　desorption,　hydrogen　temperature-programmed　reduction.　It　is　found　that　the　presence　of　chlorine　had　a　limited　impact　on　Ru　particle　size.　Residual　chlorine　originated　from　RuCl3　would　not　influence　on　Ru　3d5/2　binding　energy,　but　chlorine　from　HCl　solution　significantly　increased　Ru　3d5/2　binding　energy.　Regardless　of　their　source,　the　presence　of　chlorine　severely　reduced　the　amount　of　hydrogen　species　corresponding　to　the　desorption　peak　at　medium　temperature.　The　inhibition　effect　of　chlorine　on　hydrogen　adsorption　was　more　strong　for　Ru　catalyst　with　residual　chlorine　from　the　RuCl3　precursor.　With　a　similar　amount　of　residual　chlorine,　the　catalysts　with　chlorine　originated　from　the　RuCl3　precursor　showed　much　lower　catalytic　activity　than　those　prepared　by　impregnation　of　HCl.　These　results　suggest　that　chlorine　mainly　affects　the　catalytic　properties　of　alumina　supported　Ru　catalysts　for　ammonia　synthesis　by　selective　site　blocking.　Graphical　Abstract:　Chlorine　affects　on　the　sites　at　the　metal/support　interface　for　Ru/Al2O3　catalysts,　and　then　hydrogen　adsorption　and　ammonia　synthesis　both　are　suppressed.　©　2011　Springer　Science+Business　Media,　LLC.