Styrene-methacrylic acid copolymer [P(S-MAA)] (92/8, molar ratio) nanoparticles were prepared by a dissolution of submicron-sized P(S-MAA) particles with various molecular weights in a polyoxyethylene nonylphenylether nonionic emulsifier aqueous solution at pH 13.0 and above 90 degrees C. The latter particles were produced by emulsion copolymerization in the presence of various concentrations of n-octyl mercaptan. The effect of molecular weight on the dissolution behavior was examined at various conditions and using various kinds of the nonionic emulsifier. It was clarified that a decrease in the molecular weight led to the production of the nanoparticles at mild treatment conditions.