Substituent effects in the p pi-d system (DCNQI)(2)Cu are described. Electrical and magnetic properties of (R(1),R(2)-DCNQI)(2)Cu are quite sensitive to the substituents R(1) and R(2). Especially, when R(1) = R(2) = CH3, the system at low temperature changes from a metal to an insulator according to the isotope substitution. A variety of substituent effects would be generally understood in terms of the amount of charge transfer from Cu to DCNQI which is sensitive to the N-Cu-N coordination angle.