Electrocrystallization of [Pt(en)(2)][PtCl2(en)(2)](ClO4)(4) (1), which is a halogen-bridged mixed-valence platinum complex having Pt-II... X-Pt-IV-X ... chains, is reported. Electrochemical reduction of trans-[PtCl2(en)(2)]Cl-2 in an aqueous solution of perchloric acid at a potential of -1.5 to -1.6 V vs Pt counter electrode gave red needle crystals of (1) which are several mm long. The crystals appeared not only on the electrode but also away from it. On the other hand, electrochemical oxidation of a Pt-II complex gave no crystals of (1) and the reason for this result is discussed.