Spin-echo measurements in deuterated polyisobutylene demonstrate unambiguously that the stress relaxation and the a relaxation of the flow process are two separate processes, with a slightly but clearly different temperature dependence. Obviously, the structural relaxation of the flow process sets in only after the stress has fully relaxed. Results in protonated polyisobutylene show that the proton diffusion shares the temperature dependence of the stress relaxation, so the diffusion distances required for the flow process get longer and longer with decreasing temperature.