An infrared method developed earlier for the study of spontaneous demixing in n-alkane mixtures was used to measure microaggregation in binary mixtures of alkyl methyl esters in the crystalline state and also in phospholipid bilayers consisting of two symmetric phospholipids that differ in the length of their acyl chains. The lengths of the acyl chains involved are in the range of 18-24 carbons. Spontaneous demixing at room temperature occurred for the melt-quenched ester mixtures which, except for one case, were in the orthorhombic phase. In the exceptional case, the demixing occurred in the hexagonal phase. The kinetics of demixing are similar to those observed earlier for n-alkane mixtures. Demixing in the phospholipid mixtures was observed, but, under the conditions of our measurements, was not found to be time dependent. Our results are generally in keeping with the phase behavior reported in the literature for phospholipids with alkyl chains having 12-16 carbons. The sensitivity of the infrared method to small domains allowed us to measure aggregation in gel-state phospholipid mixtures with chain-length differences of 2 carbons, mixtures previously considered to be randomly-mixed solid solutions.