The reaction between 4-tert-butylphenol (BuPhOH) and mushroom tyrosinase was investigated by following 4-tert-butyl-ortho-benzoquinone, whose high stability permits the reaction to be used as a model for the study of the monophenolase activity of tyrosinase. The system evolves to a pseudo-steady state through an induction period (tau), the pseudo-steady-state rate (Vss) decreasing when the (BuPhOH) concentration increases. Increases in enzyme concentration result in a parabolic pattern with Vss, while tau is shortened. The addition of increasing catalytic amounts of 4-tert-butylcatechol at the start of the reaction reduces tau until it is totally abolished, an initial burst being observed at high 4-t-butylatechol concentrations. Initial bursts are also obtained at pH 4.5 or lower, indicating a lower affinity of the met-tyrosinase or oxidized form for the monophenol at low pH. These experimental results can be explained by the reaction mechanism of tyrosinase.