The relationship between liquid-liquid phase separation and microheterogeneity in water-primary alcohol mixtures was examined by analysing the mass spectra of clusters generated through the fragmentation of liquid droplets. By comparing the cluster structures of water-ethanol, -1-propanol, and -1-butanol binary mixtures at various alcohol concentrations, we discovered differences in the molecular clusters that control phase separation. We also studied the role of water in alcohol self-association. Alcohol self-association is promoted in the presence of a small amount of water (ca. 10 approximately 20 wt%), in which the water-water hydrogen-bonding network is weak and does not contribute to alcohol self-association. We have demonstrated that alcohol self-association is also promoted by non-ideal mixing with other alcohols. The self-association of alcohol molecules complements the loss of stabilization energy caused by the relatively weak coexisting interactions. This complementary relationship among intermolecular interactions is an inherent property of solutions, and plays a key role in the phase separation process.