The N/Ru(0001) system was studied by thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and high-resolution electron energy-loss spectroscopy (HREELS). Atomic nitrogen was prepared by NH3 decomposition at sample temperatures decreasing from 500 to 350 K during NH3 exposure. A maximum N coverage of theta(N) = 0.38 could thus be achieved. root 3, split 2 x 2 and 2 x 2 LEED patterns were observed for decreasing theta(N). After NH3 decomposition and before annealing the sample to a temperature above 400 K, the surface is composed elf adsorbed N, K, and NH species. This composite layer exhibits a split root 3 LEED pattern due to domains of size 4 with heavy walls, This phase decays through dissociation of NH leading to sharp first-order type desorption peaks of H-2 and N-2. From the weak intensity of the v(Ru-NH) stretch mode it is concluded that NH is adsorbed at threefold-hollow sites. The energy of the v(Ru-N) mode shifts from 70.5 to 75.5 meV when theta(N) is increased from 0.25 to 0.35.