Molecular association of malachite green (MG) in single micrometer-sized water droplets dispersed in din-butyl phthalate was studied by means of scanning laser manipulation and absorption spectroscopy to elucidate characteristic features of chemical processes at the droplet/oil interface. At a high MG concentration, the droplet showed a new absorption band around 580 nm in addition to the monomer band at 615 nm. Analyses of the concentration-dependent absorption spectrum of MG in the droplet indicated that the band at 580 mn could be assigned to that of the MG dimer. The dimer formation of MG in the water droplet was shown to be facilitated with decreasing the droplet diameter, demonstrating the micrometer size effect on dye association. A role of the water/oil interface for the MG dimer formation is discussed.