A cationic Rh (Ι) complex,[(η6-C10H8) Rh (cod)] BF4, has been found to be a quite useful catalytic system for both the intermolecular [4+ 2] cycloaddition reaction of 1, 3-dienes with nonactivated acetylenes and the intramolecular [4+ 2] cycloaddition of dienyne under very mild reaction conditions (CH2Cl2, 15° C, 15 min).