Galvanic replacement reactions provide an elegant way of transforming solid nanoparticles into complex hollow morphologies. Conventionally, galvanic replacement is studied by stopping the reaction at different stages and characterizing the products ex situ. In situ observations by liquid-cell electron microscopy can provide insight into mechanisms, rates and possible modifications of galvanic replacement reactions in the native solution environment. Here we use liquid-cell electron microscopy to investigate galvanic replacement reactions between silver nanoparticle templates and aqueous palladium salt solutions. Our in situ observations follow the transformation of the silver nanoparticles into hollow silver–palladium nanostructures. While the silver–palladium nanocages have morphologies similar to those obtained in ex situ control experiments the reaction rates are much higher, indicating that the electron beam strongly affects the galvanic-type process in the liquid-cell. By using scavengers added to the aqueous solution we identify the role of radicals generated via radiolysis by high-energy electrons in modifying galvanic reactions.