By employing a hydrogen peroxide/formic acid system, the in situ epoxidation of poly (butadiene-co-styrene) leads to controlled changes in the chemical nature as well as the molecular architecture of the original polymer. The epoxidation yield was adjusted by varying the composition of the epoxidizing agent, reaction time and temperature. As a new result it was shown that as the epoxidation content increases the reptation time (tau(rep)) is considerably shifted to lower frequencies whereas the plateau modulus (G(N)) is slightly increased. Thus, the inter-chain interaction caused by the epoxy groups over compensates the increase of the chain cross-section. Higher molecular weights shift the onset of the flow region towards lower frequencies extending the rubbery plateau modulus. The experimental data are in good agreement with predictions derived from molecular considerations of the refined tube model.