We describe a new catalyst for group IV nanowire heterostructures, based on alloying Ag with Au, that combines the ability to control catalyst phase and nanowire structure with good environmental stability. Compared to other alloy catalysts, we show a higher oxidation resistance of AgAu and more consistent crystal shapes and catalyst/nanowire orientation relationships during growth. We show that AgAu catalysts are also stable against diffusion during growth, making them capable of forming long nanowires with uniform diameters. Furthermore, we demonstrate the growth of compositionally abrupt Si/Ge heterojunctions with good reproducibility and yield, switching individual nanowires between vapor-liquid-solid and vapor-solid-solid growth to optimize growth rates by control of the catalyst state. The stability and properties of AgAu catalysts potentially open up a promising and practical route toward control of group IV heterostructure nanowires. © 2012 American Chemical Society.