We observed chromatographic separation in an open capillary tube with a water-hydrophilic-hydrophobic organic mixture carrier solution. The separation is thought to be based on the tube radial distrubution of aqueous and organic carrier solvents in the capillary tube. In this study, in order to obtain information concerning the solvent distribution, we delivered a carrier solution containing fluorescent dyes into a microchannel (100 mu m in width x 40 mu m in depth) and monitored the tube radial distribution of the dyes using a fluorescence microscope. When an organic solvent-rich carrier solution (water acetonitrile ethyl acetate; 3:8:4 volume ratio) containing perylene and Eosin Y was fed into the channel, perylene and Eosin Y were distributed around the center and near the inner wall of the microchannel, respectively. This observation consists with the specific solvent distribution proposed for the chromatography system.