The kinetics and mechanism of reduction of Cu(II) with an excess D-penicillamine have been examined at pH = 6.2 and 0.60M in ionic strength. The reaction at the initial stage is biphasic with a rapid complexation process to give "red" transient complex of $[Cu(II)(pen)_2]^2$- that is partially reduced to another transient "brown" intermediate. The "brown" intermediate is finally reduced to diamagnetic "yellow" complex, $[Cu(I)(Hpen)]_n$. The final reduction process is pseudo-first order in ["brown" transient] disappearance $with {kappa} = {{kappa}_{3a} + {kappa}_{3b}[pen]^{2-}},$ where ${kappa}_{3a} = (5.0{pm}0.8){ imes}10^{-3}sec^{-1}$ and ${kappa} = (0.14{pm}0.02) M^{-1}sec^{-1}$ at $25^{circ}C$. The activation parameters for the $[H_2pen]$-independent and $[H_2pen]$-dependent paths are ${Delta}H^{ eq} = (52{pm}5)kJmol^{-1},$ and ${Delta}S^{ eq} = ( - 27{pm}3)JK^{-1}mo^{l-1},$ and ${Delta}H^{ eq} = (56{pm}2)kJmol^{-1}$ and ${Delta} S^{ eq} = ( - 18{pm}0.7)JK^{-1}mol^{-1}$ respectively. The nature of "brown" intermediate is not clearly identified, but this intermediate seems to be in the mixed-valence state, judging from the kinetic and spectroscopic informations.