This article shows that near-infrared spectroscopy (NIRS) can be used efficiently for the simultaneous in-line and in situ monitoring of monomer (methyl methacrylate, MMA, and butyl acrylate, BuA) and polymer concentrations in the reaction medium during seeded semibatch emulsion copolymerizations. A series of actual reaction experiments was planned to allow the proper obtainment and selection of calibrating samples. Partial least squares (PLS) was used to build three independent calibration equations in the range of 1100-1900 nm, which were used to successfully monitor some disturbed reactions in-line.