The interaction between poly(butyl methacrylate) (PBMA) latex particles of different size and nonylphenol surfactants containing poly(ethylene oxide) chains (NP-EO,) of different lengths was studied by titration microcalorimetry. Three systems, 393 nm PBMA latex + NP-20, 141 nm PBMA latex + NP-20 and 393 nm PBMA latex + NP-40, were investigated. In each case the binding can be described by Langmuir type isotherms, which suggests that the dominant interaction is that of the hydrophobic portion of the surfactant with the PBMA latex surface. To differentiate the heat effects caused by adsorption from those caused by dilution of the surfactant, corrected dilution enthalpy curves were calculated. These, in turn, were used for the preparation of the net adsorption enthalpy and differential molar enthalpy of adsorption curves. The net enthalpy curves for the three systems examined have much in common. The interaction between the surfactants and the PBMA latex was endothermic over the whole range of surfactant concentrations. Thus the adsorption of NP-20 and NP-40 onto PBMA latex particles is an entropy driven process. Our results can be compared with the literature data [Int. Journal of Pharmaceutics 1993, 89, 33] on the adsorption of nonylphenol surfactants on polystyrene latex, which, unlike our systems, is characterized by a significant exothermic effect at low values of surface coverage.