The adsorption and decomposition of NH₃ and ND₃ on W{100} have been studied in detail using a molecular beam technique. Sticking probabilities and desorption rates of gases resulting from the decomposition reaction have been followed at crystal temperatures between 300 and 910 K. Isothermal desorption of D₂(H₂) was followed as a function of ND₃(NH₃) beam time on an initially clean surface. Variations in the shape of the resulting isothermal desorption curves as a function of surface temperature are explained via the changes in activation energies for desorption of D₂ and decomposition of adsorbed ND₃ as a function of N coverage. A significant kinetic isotope effect is found in H₂ and D₂ isothermal desorption during the decomposition of mixed NH₃ and ND₃ beams only at high N(a) coverages. A kinetic model and energetic scheme are presented for the ammonia decomposition and synthesis reactions on W{100} which are consistent with all of the experimental data, and show that the synthesis reaction is blocked by a large activation barrier to the initial hydrogenation of adsorbed nitrogen atoms.