In this work we study the first-order nonadiabatic corrections to Migdal–Eliashberg theory using a multiphononic model appropriate to alkali-doped fullerides. The Eliashberg spectral function, α 2 F( ω), was taken to include the most relevant phonon modes: the intermolecular, the alkali and the intramolecular ones. The critical temperature T c and the partial isotope coefficient α Rb for Rb 3C 60 are analyzed within this approach.