The adsorption of the nonionic surfactant C(12)E(7) On predominantly hydrophobic polystyrene latex particles surface-modified with aromatic amino groups has been examined as a function of C(12)E(7) concentration and pH by dynamic light scattering. The adsorption isotherms were determined by a surface tension method. At pH 6, the hydrophobic interaction between the alkyl tails of the C(12)E(7) molecules and the polymer chains dominates the adsorption. The adsorbed C(12)E(7) molecules are oriented with the POE chains extending into the solution. The C(12)E(7) molecules bridge neighboring polymer chains, causing their aggregation and thereby lowering the interfacial free energy, A thick layer of polymer chains at the latex surface allows accommodation of further C(12)E(7) molecules and finally gives a constant R(H) value. The two-stepped L-type isotherm is characteristic. When NaBr is added, hydrophobic interactions between the alkyl tails of C(12)E(7) and the polymer chain are greatly enhanced. The conformation of the polymer chains is also changed; steric hindrance due to the adsorbed C(12)E(7) molecules leads to a small extension of the polymer chains which, prior to surfactant adsorption at high ionic strength, are strongly compressed. The lower hydration of the POE chain of the C(12)E(7) at high NaBr concentration leads to a conformation which favors hydrophobic adsorption and results in a relatively large adsorbed amount at 100 mM added NaBr. The two-stepped characteristic of the L-type adsorption isotherms is unchanged when salt is added. At pH 2, the exposed amino groups at the latex surface bind strongly to the ethylene oxide unit of C(12)E(7) by hydrogen bonding, yielding a C-type isotherm. The bound POE chains close to the surface lead to steric hindrance effects and result in strong extension of the polymer chains. However, neighboring polymer chains again aggregate through bridging by the adsorbed C(12)E(7) molecules, which reduces the adsorbed amount in spite of the large initial change in the overall dimensions of the latex particle. At pH 12, the amino groups associated with OH- ions no longer interact with the ethylene oxide unit of C(12)E(7) Enhanced hydrophobic interaction then dominates the adsorption and gives an L-type isotherm with a two-stepped characteristic. Adsorption in the second step is expected to occur about the initially adsorbed C(12)E(7) molecules and finally small semiglobular micelles form along the polymer chains.