The photodesorption dynamics of ammonia from Cu(111) and GaAs(100)-(4*6) has been studied using (2+1) resonance-enhanced multiphoton ionization (REMPI) as a quantum-state-specific probe. Hyperthermal excitation of the n₂ mode is obsd. for both substrates. Photodesorption from Cu is characterized by a marked under-population of antisym. ν2 levels, whereas for NH₃/GaAs the population of the ν₂ mode follows Boltzmann statistics. The two systems also differ in the amt. of rotational excitation and translational energy. The exptl. results are discussed referring to a model which was developed recently to account for the large isotope effect in ammonia photodesorption. The model describes correctly the different quantum state distributions within the n2 manifold for the two systems.