The effect of DC8,9PC on the cooperativity of ordered-fluid phospholipid phase transitions has been investigated by determining the transitions widths in multilayer's dispersions of DXPC - DC8,9PC by DSC. The van’ t Hoff values of the main transition enthalpies were calculated using an approximate expression deduced from Zimm and Bragg theory. The DC8,9PC decreases the cooperativity (size of the cooperative unit of synthetic DXPC bilayers). The observation that the membrane lipids in the mixed lipid/DC8,9PC assemblies appear to adopt bilayer structures is important, since it demonstrates that the lipid domains within the colorimetric vesicles exist in the fundamental organizational unit found in cellular membranes. The experiments described in this work shed light upon the effects of external environmental parameters, such as temperature, upon structural and dynamical properties of the organized lipid assemblies. The colorimetric platform also facilitates elucidation of the contribution of distinct membrane components, such as cholesterol, toward shaping membrane properties. This capability could open the way for application of the assay for detailed analyses of the roles played by particular molecules, such as peptides and DNA, in determining membrane functions and properties. Between non polymeric and polymeric species in DXPC: DC8,9PC mixtures the influence of the polymer is from C14 chain-down more cooperative the non-polymeric; from C16 on before DPPC:DC8,9PC is better polymerized than non-polymerized and for DSPC:DC8,9PC there is no difference. Hypothesis is C14 or less non-polymerized are intermixed with no difference but when polymerized, non ideal mixing is formed, polymers containing polymeric units with pockets containing DMPC and/or cholesterol. When chains are C16 or more units of saturated and non saturated mixtures cooperative units are quite similar.