The direct amination of biomass-derived isohexides with NH3 over a Ru/C catalyst was systematically investigated to understand the role of H2 and NH3 in the production of isohexide diamines vs aminoalcohols, i.e., the transformation of one or both OH-groups in isohexides into NH2 groups. Only aminoalcohols with an exo-OH group were generated starting from isosorbide, which contains both an exo-OH and an endo-OH group, while a moderate yield of diamines was obtained from isomannide with two endo-OH groups due to the higher reactivity of the latter. The main byproducts were identified, including a variety of N- and O-containing cyclic compounds, such as 2,5-dimethylpyrrolidine, that arise from a decomposition path driven by hydrolysis/hydrodeoxygenation of a tricyclic amine intermediate. By combining density functional theory calculations with microkinetics, NH3 was found to adsorb strongly on the catalyst surface and generate adsorbed NH2 and NH species with variable coverage depending on the temperature and the nominal H2/NH3 ratio. Isomerization of isohexides was greatly suppressed by adsorbed NH3. Meanwhile, adsorbed NH3 discouraged the formation of byproducts driven by competing side reactions promoted by H2. The H2/NH3 ratio, which conditions the distribution of NH2 and NH species on the Ru surface, influences drastically the catalytic performance.