It is shown here that near-infrared spectroscopy (NIRS) may be used to detect the presence of monomer droplets in polymer lattices directly from spectra, without any kind of previous calibration. Experiments involving the addition of methyl methacrylate (MMA) and butyl acrylate (BuA) to monomer-free lattices indicated that there exists a specific wavelength (1620 nm) where, at some instant of the addition process, spectra are subject to a marked change. It is shown that this abrupt spectral change is caused by the formation of monomer droplets in the reaction medium. This finding may significantly improve the controllability of emulsion polymerization processes and the quality of its products, as shown by actual MMA/BuA emulsion polymerization experiments, which reinforces the potential of NIRS as a multipurpose analytic tool for in-line and in-situ simultaneous monitoring of different properties in emulsion polymerizations.