Single-layer, defect-laden hexagonal boron nitride (dh-BN) is attracting a great deal of attention for its diverse applications: catalysis on the one hand, and single photon emission on the other. As possible probes for identifying some common defects in single-layer h-BN, we present results of ab initio calculations for the adsorption and vibrational characteristics of syngas molecules (H2, CO, CO2) on dh-BN containing one of four types of defects: nitrogen vacancy (VN), boron vacancy (VB), Stone–Wales defect (SW), and nitrogen substituted by boron (BN). Through a comparative examination of adsorption features, charge transfer, electronic structure, and vibrational spectrum, we obtain a deep understanding of the interaction of these molecules with dh-BN and the role of the defect states. We find that while CO and CO2 chemisorb, molecular H2 physisorbs, but dissociative adsorption of H2 is feasible on dh-BN. VN and VB show strong affinity for CO and CO2 since the defect states induced by them lie close to the Fermi level. SW does not favor adsorption of these small molecules, as the process for each is endothermic. At BN CO adsorbs strongly but CO2 only weakly. Vibrational frequencies of notable modes localized at the adsorbed molecules are analyzed and suggested as measures for identification of the defect type. Systematically investigating the adsorption of small molecules on these defects, we predict that dh-BN with VN is a good catalyst candidate for CO2 hydrogenation.