Back potentiometric titration with addition of salt NaCl was used to determine the acid titratable percent in EHA／AA radiation emul sion copolymerized latex．It was found that

he carboxylic monomer concentration，its neutralized percent，its adding model，the emulsifier concentration，the molecular weight of copolymer have significant effects on the acid titra-table percent in polymer latex；while total adsorbed dose and dose rate have no obvious effects．In all cases investigated the acid titratable percent is above 75％，this means that most of the carboxyl groups accumulate in the latex shell of copolymer．Because the solubilitv EHA in water is very low，the micelle nucleation takes advan-rage in emulsion polymerization with EHA as main monomer．When AA concentration in water is high

the homophase nucleation becomes important．After the nucleation period，AA distributes in water and polymer-monomer par ticles by certain proportion．The AA in parti-cles copolymerizes with EHA and distributes homogeneously in polymer latex at last．While AA in water has two reaction routes：the first，diffusion into the particles，it results in that most of AA accumulates in the latex shell and the concentration of carboxyl group has a gra-client along the latex radius；the second，polymerization in water，the resulted polymer is soluble in water or is adsorbed at the latex surfac e．For unneutralized AA，the first mechanism dominates，while for partly preneutralized AA，the second mechanism takes advantage and will compete with the first mechanism for AA in water．