A new copper(II) phthalocyaninate substituted with four 15-membered tetraazamacrocycles (15M-TAMCuPc) and its pentanuclear complexes (Ni,Cu) have been studied in dimethyl sulphoxide solution by cyclic voltammetric and coulometric methods. The cyclic voltammogram of 15M-TAMCuPc is characterized by one cathodic wave which has a totally irreversible nature. However, four one-electron reduction waves which show quasi-reversible behaviour at all sweep rates were observed for the pentanuclear Cu(II) complex of 15M-TAMCuPc. On the other hand two one-electron reduction waves were observed for the pentanuclear Ni(II) complex of 15M-TAMCuPc. One of them shows reversible behaviour and the other shows quasi-reversible behaviour at all sweep rates. The reaction mechanisms and diffusion coefficients have been investigated.