Publisher Summary Alloying of a catalytically active metal with an inert component changes the selectivity in hydrocarbon reactions such that C–C bond fission is suppressed compared with C–H bond breaking. Upon alloying, the selectivity for cracking reactions decreases. This chapter shows that the surface composition of macroscopic alloys often differs from the bulk composition. Selectivity changes can be understood in terms of the primary ensemble effect—that is, the dilution of the metal surface with inactive atoms diminishes the probability for metal-adsorbate complexes containing several neighboring metal atoms as required for cracking. The concept of an ensemble of metal atoms, crucial to the understanding of selectivity patterns on alloys, implies that the metal atoms in the surface of the alloy keep their individuality and are only influenced by their immediate environment. The chapter discusses the secondary ensemble effect that ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect leads to a decrease in heat of adsorption upon alloying.