The kinetics and polymer microstructure of the seeded semibatch emulsion copolymerization of n-BA/MMA were investigated in this work. Free monomer concentration was determined online by means of FT-Raman spectrosocopy. The kinetics, gel fraction, and molecular weight distribution were determined for several semibatch reactions of different monomer composition. The experiments show that, when the amount of MMA in the recipe was increased (from 90/10 to 50/50 n-BA/MMA by moles), the instantaneous conversion increased, whereas the gel content decreased and the sol molecular weight exhibited no defined trend. A mathematical model was used to analyze the experimental data.