A B S T R A C T : As a first approach to modeling the crystallization behavior of cocoa butter, the binary system of its two major components, 1-palmitoyl-2-oleoyl-3-stearoyl-s n-glycerol (POS) and 1,3-distearoyl-2-oleoyl-s n-glycerol (SOS), was studied. Differential scanning calorimetry, coupled with polarized light microscopy, was used to determine the phase diagrams of the various polymorphic forms (the most metastable sub-! and !, intermediate " and # $, and stable #). Associated theoretical phase diagrams were also built. The presence of a solid solution for # and the ideal behavior of ! were confirmed. Kinetics of isothermal solidification as a function of temperature were studied for three compositions of the POS-SOS system. Results were displayed as time-temperature-transformation (TTT) diagrams and crystal morphology maps. Dependence of the crystallization kinetics on composition of the binary system was interpreted in terms of nucleation and growth mechanisms. The asymmetry of the POS molecule induces a slower growth rate of the mixture when the concentration of this triacylglycerol is increased. Solidification kinetics during continuous cooling were studied at various cooling rates for the system POS-SOS 25:75. Experimental results were compared with numerical predictions of a solidification model based upon TTT data and an additivity principle. The calculated and measured volume fractions of the different phases formed and times of onset and finish of the solidification were in good agreement. JAOCS 75, 857-864 (1998).1 -P a l m i t o y l -2 -o l e o y l -3 -s t e a r o y l -s n-glycerol (POS) and 1,3-distearoyl-2-oleoyl-sn-glycerol (SOS) are the two major components of cocoa butter. These products exhibit several polymorphic forms. Characteristics of the various crystalline forms of these pure triacylglycerols (TAG) (1-6), as well as their kinetic behavior (7-10), have been intensively studied. The extension of these studies to more complex mixtures is of particular interest for understanding the crystallization of a natural fat, such as cocoa butter. The binary system POS-SOS is a first step in this direction. A comprehensive review of the phase diagrams of TAG has been made by Rossel (11). Investigations done on the POS-SOS system by Wille and Lutton (12) showed a solid solution for the three phases observed, but there is some controversy about the existence of a eutectic for the # phase (13). Despite this, the most relevant diagram for this system, e.g., this measured by Wesdorp (6), confirmed the complete mixing behavior in the # phase. On the other hand, the less-stable phases of the POS-SOS system have not been completely identified and the kinetics of solidification were not characterized.In the present study, differential scanning calorimetry (DSC) coupled with polarized light microscopy experiments was used to determine the phase diagrams of the various polymorphic forms in the POS-SOS system. The kinetics of isothermal crystallization are presented in the form of time-temperature-transforma...