The highly active olefin metathesis catalyst precursors RuCl2(=CNR')(IMes)(PR3)(2) (R' = Ph, R = Cy (4), R' = CH=CMe2, R = Cpy (5)) can be conveniently synthesized from their phosphine analogues RuCl2(=CHR')(PR3)(2) (R' = Ph, R = Cy (cyclohexyl) (2), R' = CH=CMe2, R = Cpy (cyclopentyl) (3)), imidazolium salts, and a base by a facile procedure. The ligand exchange reaction is performed in a "one-pot" procedure, and an easy workup leads to good yields of 4 and 5.