Grubbs' catalyst [(PCy3)(2)Cl2Ru=CHPh, 7] was shown for the first time to be an effective catalyst for the mild intermolecular cyclotrimerization of terminal alkynes (1-6). The reaction is general and the isomeric trisubstituted benzene derivatives (8-13) were isolated in good to excellent yields. (C) 1999 Elsevier Science Ltd. All rights reserved.