Controlled synthesis of aldehyde-functional poly(methystyrene) (PMS) latexes in an emulsifier-free system was achieved via an emulsifier-free emulsion polymerization of methylstyrene using V-50 as an initiator, followed byanin-situ surface oxidation catalyzed by copper(Ⅱ) chlorideandtert-butyl hydroperoxide. Our investigation of the effect of oxidant concentration, reaction time, and temperature demonstrated that controlling the surface concentration of aldehyde and carboxylic acid groups was readily accomplished by altering these three reaction parameters. The concentration of the catalyst, however, was found to have little affect on the rate of oxidation. Furthermore, the amounts of surface aldehyde and carboxylic acid groups were determined by conductometric and potentiometric titrations. Electrolyte stability of functionalized latexes was also examined with regard to their critical coagulation concentrations in various salt solutions. Finally,SEMstudies were conducted and clearly showed that surface morphology of functional latexes hardly altered with a low degree of oxidation but became aggregates when highly oxidized latexes were formed.