We have examined the catalytic action of beta- and gamma-Mo2N in the partial hydrogenation of acetylene. The influence of variations in GHSV (230-1600 min(-1)), feed composition (5-30% v/v N-2/H-2), heating rate (0.1-5 K min(-1)) and isothermal hold (1-7 h at 933 K) on nitride structural properties has been assessed. At >= 2 K min(-1), beta-Mo2N (<= 15 m(2) g(-1)) consisting of small crystallites (< 5 mu m) was generated. At <= 0.5 K min(-1), gamma-Mo2N with a platelet morphology and surface area >= 45 m(2) g(-1) was formed. High GHSV, low N-2 feed content and a prolonged isothermal hold served to increase gamma-Mo2N area (to 135 m(2) g(-1)). Lower alkene selectivity and a twofold higher specific (per m(2)) acetylene hydrogenation rate were recorded for beta-Mo2N and linked to higher surface Mo/N ratio (from XPS). Olefin selectivity for both nitrides was greater than that reported for Pd catalysts. Moreover, we recorded negligible green oil formation in reactions over gamma-Mo2N.