AbstractSubmicron-sized styrenemethacrylic acid copolymer (92/8 molar ratio) particles dissolved in the presence of polyoxyethylene nonylphenylether nonionic emulsifier under alkaline conditions at 90C, resulting in nanoparticles with diameters of about 30nm. In order to clarify the role of the emulsifier molecules in the dissolution process, 1H NMR measurements were carried out. As the particles swelled, the NMR integrals due to the emulsifier decreased markedly. This indicates that the emulsifier molecules permeated into the inside of the alkali-swelling particles and were adsorbed on the polymer molecules, which supports the formation mechanism of nanoparticles by the particle dissolution method proposed in a previous article.