Acrylic acid (AA) is used in many emulsion polymerization formulations to improve the colloidal stability of the latex product. The improved stability originates from electrostatic repulsion complemented with steric repulsion. The strength of the electrostatic and steric repulsion forces in a styrene (S)/AA copolymer latex was investigated at different pH values, electrolyte concentrations, and temperatures. A comparison was made with an S homopolymer latex. Transmission electron microscopic pictures, combined with visual inspections, provided understanding of the mechanisms leading to coagulation in polystyrene (PS)/AA copolymer latices. Colloidal stability of the unswollen sodium dodecyl sulfate stabilized PS latex is based on electrostatic repulsion. Destabilization by sodium chloride resulted in aggregation. The acidic PS/AA latex remained stable against aggregation at high electrolyte concentrations because of steric repulsion. The acidic PS/AA latex showed a strong tendency to flocculate at increasing electrolyte concentrations. Flocculation was not observed for high-pH PS/AA latices at high electrolyte concentrations. Steric repulsion of the acid PS/AA latex was lost at temperatures higher than the critical coagulation temperature (35 degreesC), and flocculation was followed by aggregation and coalescence. The high-pH PS/AA latex was stable even at high electrolyte concentrations and temperatures up to 80 degreesC because of strong electrosteric stabilization. (C) 2003 Wiley Periodicals, Inc.