Adsorption and thermal dehydrogenation of NH3, adsorbed at 80 K on the open Ru(11(2) over bar 1) surface, was studied using high-resolution electron energy-loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). For the NH3 monolayer, a strong dipole-active mode is found at 15 meV which is newly assigned to T-z, the frustrated-translation mode perpendicular to the surface of NH3 bonding with the nitrogen atom to the Ru surface. Increasing the temperature. 70% of NH3 desorbs before a channel for dehydrogenation opens at about 280 K. The remaining 30% decomposes completely during further warming to 470 K. The dehydrogenation of NH3 gives rise to four peaks in the H-2 TDS which are assigned to desorption of coadsorbed hydrogen at 220 K and three dehydrogenation reaction steps at 320, 360. and 420 K in accordance with HREELS. The reaction intermediates NH2 and NH are identified through HREELS. In a new interpretation NH2 is characterized by intense modes at 163 meV (rocking) and at 189 meV (scissoring). Using a maximum entropy algorithm six frequencies for v(Ru-N) were resolved at 46, 50, 58, 61, 69, and 75 meV.