Apolipoprotein (apo) E contains two structural domains, a 22-kDa (amino acids 1–191) amino-terminal and a 10-kDa (amino acids 223–299) carboxyl-terminal domain. To better understand apoE-lipid interactions on lipoprotein surfaces, we determined the thermodynamic parameters for binding of apoE4 and its 22-kDa and 10-kDa fragments to triolein-egg phosphatidylcholine emulsions using a centrifugation assay and titration calorimetry. In both large (120 nm) and small (35 nm) emulsion particles, the binding affinities decreased in the order 10-kDa fragment » 34-kDa intact apoE4 > 22-kDa fragment, whereas the maximal binding capacity of intact apoE4 was much larger than those of the 22-kDa and 10-kDa fragments. These results suggest that at maximal binding, the binding behavior of intact apoE4 is different from that of each fragment and that the N-terminal domain of intact apoE4 does not contact lipid. Isothermal titration calorimetry measurements showed that apoE binding to emulsions was an exothermic process. Binding to large particles is enthalpically driven and to small particles it is entropically driven. At a low surface concentration of protein, the binding enthalpy of intact apoE4 (–69 kcal/mol) was approximately equal to the sum of the enthalpies for the 22-kDa and 10-kDa fragments, indicating that both the 22-kDa and 10-kDa fragments interact with lipids. In a saturated condition, however, the binding enthalpy of intact apoE4 (–39 kcal/mol) was less exothermic and rather similar to that of each fragment, supporting the hypothesis that only the C-terminal domain of intact apoE4 binds to lipid. We conclude that the N-terminal four-helix bundle can adopt either open or closed conformations, depending upon the surface concentration of emulsion-bound apoE.