Copolymer composition and molecular-weight distribution of linear copolymers were controlled simultaneously by maintaining concentrations of the monomers and chain-transfer agent (CTA) at calculated values. The unreacted amounts of monomers and CTA were inferred by on-line measurements of the reaction heat. Styrene/n-butyl acrylate emulsion copolymers of predefined constant compositions and MWDs (narrow and broad monomodal, and bimodal with different polydispersities index in each mode) were obtained by this control approach using tertodecyl mercaptan as CTA.