The electron states of copper have been studied by ESR and UV-Vis as a function of the method used for preparation of Cu-ZSM-5 (ion-exchange, wet impregnation, and deposition), synthesis conditions (pH of copper solution, copper precursor, temperature. etc.) and copper loading. For calcined Cu-ZSM-5 samples treated under vacuum at 400degreesC, axial ESR spectra of O- ion radicals (g(perpendicular to) = 2.05, g(parallel to) = 2.02), which are presumably assigned to linear -O--Cu+-O--Cu+-O--chains in zeolite channels, are observed. The same samples treated under vacuum at 150-400degreesC, exhibit UV-Vis absorption bands at the unexpected region of 18,000-23,000 cm(-1). They are assigned to the ligand-metal charge transfer bands in -O2--Cu2+-O2--Cu2+-O2--like chains in the zeolite channels. Along with these states, isolated octahedral Cu2+ ions with different extents of tetragonal distortion (absorption band (a.b.) 12,500-14,000 cm(-1)) and clustered copper oxide species (charge transfer band (CTB) 27,000-32,000 cm(-1)) are also observed. (C) 2003 Elsevier B.V. All rights reserved.