We report on the infrared spectra of binary di-palmitoyl-phosphatidyl-ethanolamine-cardiolipin (DPPE 1-x CLPx) monolayers and multilamellar vesicles as a function of CLP molar fraction x and temperature T. These data, which clearly show the presence of, at least, two kind of lipid domains with different thermodynamic stability and ordering of the lipid acyl chains, are consistent with similar domains observed in Langmuir-Blodgett films of the same binary system. Infrared results suggest the presence of lateral phase separation phenomena both in the bilayers and in the monolayers build up with this binary lipid mixture. These results further support the hypothesis that, within these structures, DPPE 1-x CLPx molecules, for given values of x, are organized in a superlattice as shown by thermodynamic and AFM measurements.