By a combination of neutron spin-echo and backscattering spectroscopy the a-relaxation in poly- , isoprene has been explored over an unprecedented range in momentum transfer. Corroborating and validating earlier MD-simulations, the measurements reveal a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. A consistent interpretar tion in terms of a distribution of finite jumps underlying the a-process is possible. This model leads to a time dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations.