The evolution of the molecular-weight distributions in the batch miniemulsion polymerization of styrene initiated by different oil-soluble initiators (lauroyl peroxide, benzoyl peroxide and azobisisobutyronitrile) was investigated. Two series of polymerizations were carried out. In the first, miniemulsions contained only oil-soluble initiators to minimize monomer droplet degradation by molecular diffusion. In the second, the stability of the monomer droplets was reinforced with hexadecane. Also, a series of conventional emulsion polymerizations were carried out. The effect of both the type of initiator and the extent of the compartmentalization of initiator radicals between the polymer particles on the molecular-weight distributions were analysed by means of a mathematical model. A good agreement between experimental results and model predictions was achieved.