The catalytic cycle of multicopper oxidases (MCOs) involves intramol. electron transfer (IET) from the Cu-T1 copper ion, which is the primary site of the one-electron oxidns. of the substrate, to the trinuclear copper cluster (TNC), which is the site of the four-electron redn. of dioxygen to water. In this study we report a detailed characterization of the kinetic and electrochem. properties of bilirubin oxidase (BOx) - a member of the MCO family. The exptl. results strongly indicate that under certain conditions, e.g. in alk. solns., the IET can be the rate-limiting step in the BOx catalytic cycle. The data also suggest that one of the catalytically relevant intermediates (most likely characterized by an intermediate oxidn. state of the TNC) formed during the catalytic cycle of BOx has a redox potential close to 0.4 V, indicating an uphill IET process from the T1 copper site (0.7 V) to the Cu-T23. These suggestions are supported by calcns. of the IET rate, based on the exptl. obsd. Gibbs free energy change and theor. ests. of reorganization energy obtained by combined quantum and mol. mech. (QM/MM) calcns.