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The epoxidation of isoprene (2-methyl-1,3-butadiene) and piperylene (1,3-pentadiene), both conjugated olefins containing allylic methyl groups, has been conducted using conventional, CsCl-promoted, Ag/α-Al2O3 catalysts. Selectivities to the allylic olefin epoxide isomers are over 20% and are much higher than expected due to the presence of the conjugated olefin structure. The epoxidation of propylene over the same catalyst under the same conditions is only 2.5%. The epoxidation of propylene in the presence of butadiene also yields PO in much higher selectivities. The presence of as little as 1% C4H6 in the reaction feedstream increases the selectivity to PO from 2.5% to over 40% at the expense of overall activity for C3H6 conversion. The upper limit of selectivity to PO in the presence of C4H6 appears to be approximately 50%, suggesting an upper limit for the effectiveness of this methodology. Epoxidation of C4H6 alone on similar Ag catalysts indicates that the consecutive reaction of EpB to CO2/H2O is strongly limited by the presence of excess C4H6 in the feedstream. In addition, the selectivity to EpB is directly proportional to the amount of C4H6 in the reaction feed stream. Selectivities >90% are obtained only when there is sufficient C4H6 in the reaction feedstream to control the concentration of the reactive Ag–O surface. For C4H6 epoxidation, all CO2/H2O is formed by a consecutive reaction pathway from EpB; there is no parallel pathway for the direct formation of CO2/H2O from C4H6. Using the selective epoxidation of C4H6 as the model for understanding the enhancement in selectivity for allylic olefin epoxide formation, the most likely reason for improved selectivities is that strongly adsorbed C4H6 (or other conjugated olefins) limits the ensemble size of contiguous Ag–O surface sites. These ensembles are too small for PO combustion, but not too small for PO formation.