Oxidative ethylene hydrophenylation catalyzed by palladium(II) acetate with Cu(II) oxidants to produce styrene generally suffers from low selectivity and/or low yield. Commonly observed side products include vinyl carboxylates and stilbene. In this Article, the selectivity for styrene formation by Pd(OAc) 2 is studied as a function of reaction temperature, ethylene pressure, Brønsted acid additive, Cu(II) oxidant amount, and oxygen pressure. Under optimized conditions, at high temperatures (180 °C) and low olefin pressure (20 psig), nearly quantitative yield (>95%) of styrene is produced based on the limiting reagent copper(II) pivalate. We propose the selectivity for styrene versus vinyl pivalate at 180 °C is due to a palladium-catalyzed conversion of benzene and in situ formed vinyl pivalate to styrene.