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Cyclopentadiene was hydrogenated to cyclopentene selectively by using colloidal palladium supported on chelate resin with iminodiacetic acid moieties as a catalyst. The catalyst, containing metal particles of 1–6 nm in diameter, was prepared by reduction of the palladium(II)–chelate resin complex with methanol. Cyclopentene was produced in 97.1% yield at 100% conversion of cyclopentadiene at 30 °C under 1 atm of hydrogen. This value in yield was also achieved on repeated uses of the catalyst. The hydrogenation rate was correlated to the polarity parameter, ET(30) values, of the solvents used in the reaction, except in the case of dimethyl sulfoxide. The hydrogenation rates of cyclopentadiene (RD) and cyclopentene (RE) were expressed as RD=kD[H2][catalyst] and RE=kE[H2][cyclopentene][catalyst], respectively, where kD and kE are the rate constants. The equilibrium constant for complex formation between cyclopentadiene and the catalyst, KD, was estimated to be over 400 times larger than that between cyclopentene and the catalyst, KE. A mechanism, including the coordination of olefins to the catalyst and the subsequent hydrogenation of the coordinated complexes, was proposed.