The homogeneous vs heterogeneous contributions when using solid catalysts for the Suzuki-Miyaura coupling is still disputed. Leaching is often observed and quantified albeit with unclear conclusions about contributions of the leached species and of the solid catalyst to the global catalytic activity. In this work, a new flow reactor design to discriminate both contributions is proposed. With the help of a simple reactor model, it has been possible to conclude that the coupling of 4iodoacetophenone with phenylboronic acid proceeded with the leached homogeneous species only, whatever the solid Pd/silica used, whereas chloro-derivatives behaves differently. This reactor is simple to build and could be of general use to reveal actual heterogeneous vs homogeneous catalysis for many reactions. Keywords Suzuki-Miyaura • Heterogeneous • Leaching • Palladium The knowledge of the reaction location is of prime importance both to design efficient industrial processes and to understand the underlying fundamental mechanisms. For example, many catalytic processes are operated with heterogeneous (solid) catalysts for easier separation and downstream treatments thus to lower operating costs. However, when the fluid phase is a liquid, it appears that the knowledge of the reaction location is not straightforward. Indeed, active catalytic species may leach from the solid, which is then a simple catalyst precursor, and solubilizes in the liquid phase in which it catalyzes the reaction. Several papers including recent reviews have analyzed such situations for many different type of catalytic processes.[1,2] One must realize that the design of a catalytic reactor is largely based on the assumption of the reaction location simply because the reaction time, i.e. the contact time between the

A critical review of conclusions about the putative heterogeneous mechanism in the Suzuki-Miyaura coupling by supported Pd solids is reported. In the first section, the turnover frequencies (TOF) of 20 well-established homogeneous catalysts are shown to be in the range 200 to 1,000,000,000 h − 1 . The evidences used to prove a heterogeneous mechanism are discussed and another interpretation is proposed, hypothesizing that only the leached species are responsible for the catalytic reaction, even at ppb levels. Considering more than 40 published catalytic systems for which liquid phase Pd content have been reported, activities have been computed based on leached Pd concentrations and are shown to be in the range TOF 150 to 70,000,000 h − 1 . Such values are compatible with those found for the well-established homogeneous catalysts which questions the validity of the conclusions raised by many papers about the heterogeneous (solid) nature of Suzuki-Miyaura catalysis. Last, a tentative methodology is proposed which involves the rational use of well-known tests (hot-filtration test, mercury test…) to help to discriminate between homogeneous and heterogeneous mechanisms.