The behaviour of phosphine substituted ruthenium carbonyl carboxylates in the presence of hex-l-ene has been followed by IR and NMR spectroscopies. The complex Ru-4(CO)(8)(MeCO2)(4)(PBu3)(2) reacts at room temperature with a large excess of hex-1-ene giving the Ru-2(CO)(4)(MeCO2)(2)(PBu3)(hex-1-ene) compound. The same complex is formed from Ru-2(CO)(4)(-)(MeCO2)(2)(PBu3)(2) and hex-1-ene at 80 degrees C. Mononuclear Ru(CO)(2)(MeCO2)(2)(PBu3)(2) is not transformed under the same conditions. Catalytic tests performed at 80 degrees C in the presence of Ru-4(CO)(8)(MeCO2)(4)(PBu3)(2) or Ru-2(CO)(4)-(MeCO2)(2)(PBu3)(2) indicate that these complexes display almost the same catalytic activity in hex-1-ene isomerization in agreement with the formation of the same intermediate. Working in the presence of Ru-4(CO)(8)(MeCO2)(4)(PBu3)(2) at 80 degrees C, an 85% conversion was obtained after 70 h. The isomeric olefins had a trans/cis ratio of 4.5. A reaction scheme has been suggested to rationalise the behaviour of these complexes.