It is shown that the large deviations of the intermediate scattering function S(Q,t) from the prediction of the Rosue model as observed for polyisobutylene (PIB) chains in the melt cannot be explained by the stiff chain model. By presenting new data on polydimethylsiloxane with the same structural parameters but low internal barriers which follow the Rouse prediction over the full Q-range, the conclusion is reached that the PIB dynamics is slowed down by internal friction. © 2000 American Institute of Physics.