The dynamics of predissociating Rydberg electron wave packets are observed using the optical Ramsey method. The time-resolved spectra are hydrogenic and are very well modeled by assuming that only one p Rydberg series contributes to the dynamics. This is in contrast with previous observations of autoionizing Rydberg electron wave packets [Phys. Rev. Lett. 83, 2552 (1999)], which show quite dramatic deviations from hydrogenic behavior above the Born-Oppenheimer limit. The origin of these deviations lies in the interplay between electronic and molecular phase. By exploiting these phases we are able to control the ratio of predissociaton to autoionization. (C) 2003 American Institute of Physics.