Oil-in-water microemulsions which contain a hydrophobic colorant were evaluated as water based ink jet inks.These microemulsion based ink jet inks are thermodynamically stable, have the features of dye based inks prior to printing, and the features of pigment based inks after printing. The microemulsion was prepared and printed, as described recently by Magdassi et al.^1,2 using the gemini-type surfactant, didodecyldiphenylether disulfonate, toluene, 1-propanol, water as the continuous phase and Nile Red or Sudan IV as the hydrophobic dye. Microemulsion average droplet size was measured by dynamic light scattering and found to be ∼8 nm. We found by AFM imaging that the printed 20 – 60 μm droplets are composed of nanoparticles with average size of 130 nm. The printing process was evaluated by a fluorescence microscope, while images of the droplets were viewed as a function of time after impact with the substrate. It was found that the microemulsion droplets formed distorted spheres on ink jet paper, more perfectly shaped spheres on Forbo paper substrate and perfect round spheres on vinyl slides. When printed on glass, a ‘bagel like’ shape was obtained. The droplet size varied, depending on the surface energy of the substrate, being at least 20 μm for low energy surfaces and growing larger for higher energy substrates. It was noticed that the time scales for spreading on paper was less than tens of milliseconds accompanied and followed by fixation and drying of the droplets in less than 3 – 5 seconds.