A model is presented for the asdorption of a mixture of anionic and a nonionic surfactant on latex particles. The model is based on the Langumir equation for adsorption and the regular solution theory for mixed micelle formation. Good agreement between the experimental data and the calculated adsorption was obtained with amixture of sodium dodecyl sulphate (SDS) and nonylphenol ethoxylate (NPE10). The preferential adsorption of the more surface active NPE10 is greatest at the onset of micellization and diminshes as the total surfactant concentration increases. The composition of the mixed surfactant at the surface gratly depends on the total surfactant concentration and on the surface area/solution volume ratio. This is illustrated by the calculated adsorption equilibria at differnet conditions. Addition of small amounts of organic solvents (e.g.,coalescing aids) increases the critical micelle concentration (cmc)of NPE10 and decreases the cmc of SDS. the decreade in the difference between the cmc values of the two surfactants is paralleled by a reduction in the preferential adsorption of the nonionic surfactant in the presence of a coalescing aid.