Optical transitions in normal-spinel Co 3 O 4 have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co 3 O 4 by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution รฐZn x Co 32x O 4 ร can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d -d charge-transfer transition between the t 2g states of octahedral Co 3รพ ion and t 2 states of tetrahedral Co 2รพ ion, t 2g รฐCo 3รพ ร ! t 2 รฐCo 2รพ ร: The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O 22 ion: pรฐO 22 ร ! t 2 รฐCo 2รพ ร for the 2.4 eV absorption and pรฐO 22 ร ! e g รฐCo 3รพ ร for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for Zn x Co 32x O 4 can be explained in terms of the substitution of the tetrahedral Co 2รพ sites by Zn 2รพ ions. The crystal-field splitting D Oh between the e g and the t 2g states of the octahedral Co 3รพ ion is estimated to be 2 eV.