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Presently, mechanistic paradigms in catalysis generally posit that the active species remains either homogeneous or heterogeneous throughout the reaction. In this work, we show that a prominent industrial process, palladium (Pd)–catalyzed vinyl acetate synthesis, proceeds via interconversion of heterogeneous Pd(0) and homogeneous Pd(II) during catalysis, with each species playing a complementary role. Using electrochemical probes, we found that heterogeneous, nanoparticulate Pd(0) serves as an active oxygen reduction electrocatalyst to furnish the high potential required for corrosion to form homogeneous Pd(II), which then catalyzes selective ethylene acetoxylation with reformation of heterogeneous Pd(0). Inhibiting the corrosion of Pd(0) to Pd(II) by galvanic protection results in reversible poisoning of catalysis, evincing the essential role of phase conversion in this catalytic cycle. These results highlight how dynamic phase interconversion can harness and couple complementary reactivity across molecular and material active sites.