A time-resolved SANS technique was elaborated to study the unimer exchange kinetics in block copolymer micelles under equilibrium conditions exploiting the large difference in scattering length density between deuterated and protonated species. The technique was applied to the system PEPPEO/DMF where in contrast to theoretical predictions the observed kinetics consists of two well separated processes. While the fast process can be associated with a single unimer exchange the second slow process could not yet be assigned to a microscopic picture.