The oil-in-water microemulsion copolymerizations of butyl acrylate and acrylonitrile initiated by water (ammonium peroxodisulfate, AP)-and oil (dibenzoyl peroxide, DBP)-soluble radical initiators were investigated. Copolymerizations show two distinct nonstationary rate regions. The maximum rate of polymerization is found to be proportional to the 0.48th and 0.65th power of the AP and DBP concentration, respectively. The rate per particle is found to be proportional to the 0.05th and 0.2nd power of the AP and DBP concentration, respectively. The rate of polymerization decreases with increasing the acrylonitrile concentration. The number of particle increases with increasing conversion up to 50-70%. The number-average molecular weight increases with conversion up to ca. 20% and then decreases. The number-average molecular weights were found to decrease with increasing the concentration of both initiator and acrylonitrile. The experimental results were discussed in terms of the water-phase polymerization, the chain-transfer and radical desorption events, the particle nucleation during the whole polymerization, and recruiting monomer and emulsifier from the free monomer-swollen emulsifier micelles.