In order to explain the experimentally observed saturation swelling of ABS latex particles by a styrene (St) and acrylonitrile (AN) monomer mixture, a two-phase swelling model is proposed based on the assumption that, in ABS latex particles, poly(styrene-co-acrylonitrile) (SAN) domains are randomly dispersed in a continuous polybutadiene (PB) matrix and that thermodynamic equilibrium for both styrene and acrylonitrile monomers is attained among the SAN domain, PB matrix, and monomer droplet; phases, respectively, It is shown that the observed saturation concentrations of St and AN monomers in ABS latex particles consisting of different weight ratios of PB/SAN agree very well with those predicted by this model.