Ruthenium based catalysts are versatile promoters of a large variety of reactions. A catalytic system active in metathesis has been generated in situ from [RuCl2(p-cymene)]2, 1,3-bismesitylimidazolinium chloride, as precursor of a bulky carbene ligand, and cesium carbonate. We report that this three component catalytic system exhibits dichotomous reactivity for the transformation of dienes, providing