In this work, H-1 NMR was used to examine the anionic copolymerization kinetics of ethylene oxide and 1,2-butylene oxide. The in situ NMR technique allows monitoring the concentration profiles of both monomers simultaneously. A series of polymerization experiments at different monomer and initiator concentrations were done in order to determine the copolymerization rate constants The data were evaluated by fitting the result of a numerical solution of the kinetic differential equations to the NMR data. This procedure allowed calculating all four rate constants, k(EE), k(EB), k(BE), and k(BB), individually instead of the commonly determined reactivity ratios r(E) = k(EE)/k(EB) and r(B) = k(BB)/k(BE). The monomer incorporation into the copolymer chains is dominated by the different reactivities of the monomers, whereas the nature of the chain ends is of minor importance. In the system investigated ethylene oxide is about 6.5 times more reactive than 1,2-butylene oxide. The compositional profiles of the final copolymers can be calculated from the time resolved concentration profiles If both monomers are present at the start of the polymerization the compositional profiles have a sigmoidal shape with one chain end containing mainly ethylene oxide and the other chain end being formed almost exclusively of butylene oxide units. However, with the knowledge of the copolymerization rate constants it is possible to realize other compositional profiles. If the reactor is first charged with ethylene oxide the addition rates of butylene oxide can be calculated in order to obtain any other arbitrarily chosen compositional profile.