Abstract In the presence of small amounts of [Ru(η 4-COD)](η 6-C 8H 10)] (1), cycloheptatriene is hydrogenated to cycloheptene, under one atmosphere of hydrogen at room temperature in homogeneous phase. The formation of a small amount of cyclooctene and the existence of an induction period, which do not occur when [Ru(η 4-COD)(η 6-C 7H 8)] (2) is used as the catalyst, suggest that 2 is the real catalyst. The selectivity of this hydrogenation is 100% in n-hexane as solvent, 99.5% in THF, and low in ethanol. Conversion is quantitative in THF and ethanol, but not more than 65% in n-hexane. In the presence of 1 or 2, cycloheptene is rapidly hydrogenated to cycloheptane in THF and ethanol, but not in n-hexane. A mechanism for these catalytic hydrogenations is proposed, and discussed on the basis of the dominant role of the solvents. Increase of temperature and/or pressure of hydrogen increases the rate of hydrogenation.