We present experimental evidence for a deceleration of convoy electrons produced by 5 MeV/u ions (N7+, Ne10+, S13+, Ni23+, Ag37+) during the interaction with insulator foils at normal incidence. The deceleration first increases with increasing projectile charge, reaches a maximum at a projectile charge of about 16, and seems to approach zero for even higher charges. Different possible mechanisms and quantitative estimates for the slowing down of convoy electrons are presented.