A systematic study of new additives for the stoichiometric molybdenum hexacarbonyl-mediated Pauson–Khand reaction resulted in the discovery of several active compounds such as tetra-substituted thioureas, ester and amide derivatives of phosphoric acid, quaternary ammonium bromides and phosphine oxides. Tributylphosphine oxide (TBPO) was the most efficient additive providing PK products in moderate to good yields. Some experimental evidences were found which support a non-oxidative Mo(CO) 6 activation by TBPO.