Polarization-modulated Fourier-transform infrared reflection absorption spectroscopy has been applied to the characterization of thin films of AlN and Al oxynitride ('AION') on NiAL(111) and to the observation of surface chemical processes under steady-state conditions in a static NH3 ambient of up to 200 Torr. AN films, grown by dosing NiAl(111) with NH3 at similar to 1230 K, are found to be structurally inhomogeneous and largely disordered, with no evidence of chemisorption at 300 K in a background of 200 Torr of NH3. AlON films, grown by dosing NiAl(111) with NO near 300 K and annealing to 1000 K, appear more homogeneous and show distinct surface and bulk vibrational modes. The former responds reversibly to chemisorbed NH3 which can be detected under steady-state conditions by the appearance of the delta(s) symmetric deformation mode. In comparison to similar results for Al2O3, the delta(s) frequency indicates a lesser degree of Lewis acidity for the AlON surface.