Cu X-ray absorption edge features of 19 Cu (I) and 40 Cu (II) model complexes have been systematically studied and correlated with oxidation state and geometry. Studies of Cu (I) model complexes with different coordination numberreveal that an 8983-8984-eV peak (assigned as the Cu Is-* 4p transition) can be correlated in energy, shape, and intensity with ligation and site geometry of the cuprous ion. These Cu (I) edge features have been qualitatively interpreted with ligand field concepts. Alternatively, no Cu (II) complex exhibits a peak below 8985.0 eV. The limited intensity observed in the8983-8985-eV region for some Cu (II) complexes is associated with the tail of an absorption peak at~ 8986 eV which is affected by the covalency of the equatorial ligands. These model studies allow accurate calibration of a normalized difference edge procedure which is used for the quantitative determination of Cu (I) content in copper complexes of mixed oxidation state composition. This normalized difference edge analysis is then used to quantitatively determine the oxidation states of the copper sites in type 2 copper-depleted (T2D) and native formsof the multicopper oxidase, Rhus vernicifera lacease. The type 3 site of the T2D lacease is found to be fully reduced and stable to oxidation by 02 or by 25-fold protein equivalents of ferricyanide, but it can be oxidized by reaction with peroxide. The increase in intensityof the 330-nm absorption feature which results from peroxide titration of T2D lacease is found to correlate linearly with the percent of oxidation of the binuclear copper site. Thiscorrelation indicates that peroxide oxidizes but does notbind to the T3 site. We have used thiscorrelation to determine that native lacease, as isolated, contains 22±3% reducedT3 sites and that all spectral changes observed upon peroxide addition to native lacease can be accounted for by oxidationof these reduced sites. In the presence of azide and peroxide, further reduction occurs and as much as 40% of the binuclear copper pairs are stabilized in the reduced state. The importance of these results to previous reports of peroxide binding at the lacease active site is discussed.