The thermodynamic and morphological properties of binary films of phospholipids and diacetylene lipids deposited at the air-water interface have been studied. Brewster angle microscopy (BAM) and fluorescence microscopy have been applied to investigate the formation, organization, and structure of the film domains. BAM data acquired at different temperatures, film compositions, and surface pressures reveal the appearance of distinct patterns of the diacetylenic moieties. In particular, the exceptionally high-quality BAM images point to dendritic and fractal-like appearance of the diacetylene domains; these phenomena are discussed in a framework of diffusion-limited aggregation processes. The results of the microscopy analyses further indicate that the two molecular components already segregate at low compression pressures and that the combined effects of film composition and temperature influence the occurrence of transitions between different phases within the films. This study sheds light on the molecular and cooperative features of mixed lipid/diacetylene films and further helps to understand the unique biosensing properties of these assemblies.