AbstractThe unbound bilirubin concentration and the enzymatic rate of bilirubin degradation by bilirubin oxidase in bilirubin‐serum albumin solutions have been investigated experimentally and theoretically. A stoichiometric bilirubin‐serum albumin binding analysis shows that the unbound bilirubin concentration depends only on the molar ratio of the total bilirubin concentration to the total serum albumin concentration. From the theoretical analysis and the measured unbound bilirubin concentrations, serum albumin may be modelled as a molecule having two binding sites, primary and secondary, with stoichiometric equilibrium constants of K1 = 6 × 107M−1 and K2 = 4.5 × 106M−1, respectively. The rate of total bilirubin degradation in bilirubin‐serum albumin mixtures is zero order. An immobilized bilirubin oxidase reactor model, which shows good agreement with experimental bilirubin conversions, is presented. At a flow rate of 1 mL/min with a 8‐mL reactor volume, a 50% bilirubin conversion per pass was observed with an inlet bilirubin concentration of 350μM and a serum albumin concentration of 500μM.