Kinetic properties of a Pt/lambda-MnO2 electrode for the electrochemical insertion of lithium ions were studied in the aqueous phase by cyclic voltammetry (CV) and potential step chronoamperometry (PSCA). The cyclic voltammogram showed that the electrochemical insertion reaction of lithium ions proceeds in two steps in the potential range between 0.60 and 0.98 V (vs, saturated calomel electrode). The chemical diffusion coefficients of lithium ions in the manganese oxide were evaluated from the PSCA data using a finite diffusion model. They were in the range of 6.6 x 10(-11) to 1.4 x 10(-10) cm(2) s(-1) depending slightly on the applied potential. The activation energy of the diffusion was evaluated as about 44 kJ mol(-1) from the temperature dependence of the PSCA. These results suggested that lithium ions diffuse by a hopping mechanism in the spinel phase. The diffusion coefficients, transfer coefficients, and standard rate constants also were estimated from the CV data. The diffusion coefficients agreed reasonably well with those obtained by PSCA.