The results of comparative kinetic experiments, homogeneity/heterogeneity testing data, and the relative substrate reactivity data for the Suzuki and Heck reactions are consistent with the participation of heterogeneous forms of the catalyst (colloidal palladium and larger palladium aggregates) in the Suzuki reaction. This is the reason why the Suzuki reaction occurs more readily than the Heck reaction. The data obtained in this study indicate that these reactions differ in the nature of catalyst deactivation processes.