As Chance et al. [1-3] proposed, the decomposition of hydrogen peroxide catalyzed by catalase is an overall first-order reaction. In this paper, we have studied this enzyme-catalyzed reaction with a thermokinetic method. The rate constant and the molar reaction enthalpy of this reaction have been measured. At 310.15 K and pH=8.2, k(cat)=1.75x10(6) l mol(-1) s(-1) Delta(r)H(m)=88.99 kJ mol(-1). Furthermore, we have studied the competitive inhibition of catalase by cyanide ion and reported some correlated parameters.