The “shuttling” kinetics of Pd mass transfer (from solid phase to solution phase and back to solid phase) have been verified by three-phase test experiments, while the solution-phase catalytic activity which characterizes most heterogeneous cross-coupling has been verified by TEM, hot filtration, and poisoning experiments. However, truly heterogeneous cross-coupling systems may exist. Poyatos et al. immobilized a Pd pincer carbene complex (Figure 3) on MK-10 clay and observed that while high TON (ca. 10) and TOF was maintained relative to the soluble catalyst, no activity was found in the solution for the supported catalyst – a strong indicator of a fully heterogeneous catalytic mechanism.