Copolymers of N-vinylformamide and acrylic acid were synthesized by conventional aqueous free-radical polymerization. The phase behavior of the copolymer solutions was investigated through the addition of hydrochloric acid and the variation of the temperature. With a moderate content of N-vinylformamide, the copolymers showed complex phase behaviors. Under low-acidity conditions, a suspending liquid was formed, whereas under high-acidity conditions, the random copolymers could assemble into round nanoparticles with a broad particle size distribution. When the temperature was reduced, a much less cloudy solution that contained nanoparticles became a milk-like suspending liquid, and this temperature-responsive phenomenon was reversible. This indicated that the formamide group was protonated under highly acidic conditions; the repulsive electrostatic forces compelled the aggregates to disperse. The amount of salt and excess acid influenced the particle size as well as the temperature-responsive behavior of the copolymers under highly acidic conditions. (C) 2008 Wiley Periodicals, Inc.