By means of recording a simple serie of merocyanine 540 spectra, we present a method to calculate the value proportional to co-operative unit size of membranes (n). Our calculations, applied to different liposomal samples processed in the presence or absence of sugars, in high or low ionic strength showed two main results. First, that any temperature cycling in high ionic strength of rigid DPPC bilayers will modify the membrane cooperativity. Second, the presence of polysaccharide Neu-5-ac in solution will always produce a strong drop in co-operativity of a rigid membrane of DPPC, whenever the negative charge is fully exposed. This last result indicates a differential ability of charged Neu-5-ac to disrupt a rigid membrane structure, even in the absence of a covalent linkage and--remarkably-in fully hydrated media.