The effects of seven sintering aids (0.5 at.% Ni, Co, Fe, Cr, Zr, Nb, and Pd) on the densification of Mo-Si-B alloys of six different compositions (Mo, Mo-0.2Si, Mo-0.2Si-0.02B, Mo-2.5Si-2.5B, Mo-7Si-5B, and Mo-8.9Si-7.7B at.%) are systematically investigated. It was found that Ni, Co, and Fe are effective in enhancing densification of Mo-Si-B alloys, and Ni is the most effective sintering aid. This study supports a previously proposed hypothesis that activated sintering results from enhanced mass transport in the sintering-aid-induced quasi-liquid intergranular films (a type of grain boundary complexion). The relative effectiveness of these sintering aids can be rationalized by analyzing several key thermodynamic parameters that control the stability of premelting-like grain boundary complexions. Future studies are needed to develop interfacial thermodynamic models and methods for computing "grain boundary complexion (phase) diagrams" for multicomponent systems, which can be a useful component for the "Materials Genome" project that will enable better predictions of the activated sintering and other materials phenomena.