L-Arabinose ketol-isomerase (EC 5. 3. 1. 4) was extracted from the L-arabinose-grown cells of Clostridium acetobutylicum and partially purified by ammonium sulfate fractionation, DEAE-cellulose column chromatography and the gel filtration on Sephadex G-200. Optimum pH and temperature for the activity of the enzyme were 7.5 and 37°C, respectively. The enzyme was inhibited by EDTA, Na-pyrophosphate and KCN, and its activity was restored by the addition of divalent metal ions such as Mn2+ and Co2+. The Michaelis-Menten's constants for L-arabinose, Mn2+ and Co2+ were 1.08×10-2M, 0.83×10-4M and 0.38×10-4M, respectively. The equilibrium constant (K=ribulose/arabinose) was 3.14 (37◊C, pH 7.5). The thermodynamic quantities, ΔH, ΔG (37°C), ΔS (37°C) and E were +7354cal/mole, -710.7cal/mole, +26.0cal/(deg•mole) and +7471cal/mole, respectively.