Brewster angle microscopy (BAM) is applied for studying the in-plane morphology of a cationic lipid monolayer and its interaction with a novel, water-soluble, J-aggregate forming, negatively charged carbocyanine dye. A methodology of monolayer compression/recompression is followed to influence the dye crystallization upon adsorption from an aqueous subsolution at a charged interface. Crystalline domains of a dendritic type are observed in the lipid monolayer compressed on a pure water subphase. Dye aggregation and crystalline nucleation is established by means of BAM for 2D lipid densities corresponding to the onset of the main phase transition of the composite monolayer. Subphase conditions, which prevent the nucleation of lipid 2D crystalline phase domains, are found to not favor dye crystallization at the interface, Dye adsorption and aggregation is established to be partially irreversible upon monolayer decompression. Dye desorption is related to the observation of stripe-like objects in the lipid monolayer, developing undulation instability, which produces non-equilibrium 2D labyrinthine patterns with time.