Catalysis　is　generally　explained　by　the　reaction　of　adsorbed　species,　but　in　certain　cases　it　can　be　also　understood　by　the　involvement　of　surface　＂quasi-compounds＂.　A　quasi-compound　is　a　combination　of　elements　that　can　exist　only　on　a　specific　crystalline　surface,　but　cannot　be　taken　out　from　the　surface.　A　prominent　example　is　found　in　the　ammonia　synthesis　reaction.　Decomposition　of　NH3　occurs　on　most　metals　and　its　kinetics　is　governed　by　the　rate-determining　desorption　of　N-2,　but　the　synthesis　reaction　occurs　only　on　a　limited　number　of　clean　metals.　Precious　metals　in　pure　form　are　inactive　for　the　ammonia　synthesis　reaction,　however,　the　reaction　of　adsorbed　N(a)　with　H-2　on　precious　metals　gives　NH3　via　NH(a).　Among　base　metals　Ni　is　also　inactive　for　the　ammonia　synthesis　reaction.　But　N-atoms　form　a　quasi-compound　Ni3N　on　Ni(110)　rather　than　adsorbed　species,　and　the　hydrogenation　of　Ni3N　with　H　gives　NH3　via　the　NH(a)　intermediate.　On　the　other　hand,　adsorption　of　NH3　gives　only　NH2(a)　on　these　metals.　Interestingly,　Ni3N　on　Ni(110)　becomes　inactive　for　reaction　with　H-atoms　at　temperatures　higher　than　approximately　500　K,　because　the　quasi-compound　Ni3NH　undergoes　decomposition　to　Ni3N　at　approximatelt　500　K;　that　is,　no　NH2(a)　is　formed　at　temperatures　higher　than　500　K.　A　large　inverse　hydrogen　isotope　effect　(r(D)/r(H)　=　2-3)　has　been　reported　for　the　ammonia　synthesis　reaction　on　a　doubly　promoted　Fe-catalyst,　and　interpreted　on　thermodynamic　grounds.　Therefore,　we　propose　an　alternative　explanation　for　this　inverse　isotope　effect　based　on　the　fact　that　the　reaction　of　N(a)　on　the　precious　metals　with　H-2　+　D-2　(H-2/D-2　=　1)　gives　equal　amounts　of　NH(a)　and　ND(a).　As　discussed　in　this　review,　the　contribution　of　labile　quasi-compounds,　[FexN]　and　[FexNH],　is　useful　in　this　regard,　and　accounts　for　the　formation　of　NH3　by　the　hydrogenation　of　an　NH2　intermediate　formed　from　[FexNH].　According　to　this　mechanism,　the　activity　of　the　Fe-surface　is　given　by　the　steady　rate　of　formation　of　labile　[FexRN]　and　[FexNH]　to　give　adsorbed　NH2(a)　on　the　Fe-surface.