Langmuir-Blodgett monolayers of L-alpha-dipalmitoylphosphatidylcholine (DPPC) doped with 1,1＇-dioctadecyl-3,3,3＇,3＇-tetramethylindocarbocyanine perchlorate (diIC(18)) are studied under conditions of high relative humidity (RH). Monolayers transferred to a freshly cleaved mica surface in the liquid expanded (LE)-liquid condensed (LC) phase coexistence region of the pressure isotherm are studied with tapping-mode atomic force microscopy (TM-AFM), near-field scanning optical microscopy (NSOM), and confocal microscopy. As the RH is increased above 65%, the small LE and LC domains become mobile on the surface and aggregate to form larger domains of like phase. Analysis of the AFM images confirms that this process does not reflect phase transitions in the monolayer at increased RH but is driven by the energetically favorable decrease in the interfacial line energy of the lipid domains upon aggregation. NSOM measurements of the monolayer confirm that the evolution in the monolayer structure at high RH does not involve defect formation, collapse of the film, or removal of lipid. The evolution in monolayer properties at increasing RH is also found to be very sensitive to the composition of the subphase used in the film transfer process, which may be important for practical applications of these films.