A method to determine the minimum time monomer addition policy for composition control in unseeded emulsion polymerization systems based on calorimetric measurements was developed. The iterative approach requires a series of semicontinuous emulsion copolymerizations to be carried out under semistarved conditions. Only the values of the reactivity ratios and the heat of homopolymerization of each monomer involved are required to apply the approach. The method, which was checked in the vinyl acetate-butyl acrylate emulsion copolymerization, converged in four steps even when technical grade monomers were used.