The title molecule, [Co3(C2H3)(C5H9N)3(CO)6] or [Co3(3-CCH3)(CNtBu)3(CO)6], lies on a threefold rotation axis. The three isonitrile ligands each occupy an equatorial site on each of the three Co atoms. The average Co-Co bond length is 2.4769 (6) Å. The tert-butyl groups are disordered over two orientations, with site occupancies of ca 0.6:0.4.