Small amounts of dye vapors are mixed with helium in a positive column of a direct‐current discharge. The distributions of dye vapor from a central reservoir towards the anode and towards the cathode are asymmetrical due to cataphoresis. In addition to this, the concentration of dye vapor along the tube axis sharply decreases with distance form the reservoir due to dissociation by electronic collision. From the comparison between experimental and calculated results the degree of ionization &agr; is estimated to be 10−3∼10−4for 1,4‐Di‐[2‐(5‐phenyloxazolyl)]‐benzene (POPOP) and 3×10−3for perylene, and the cross section for dissociation by electronic collision &sgr;dis estimated to be 2×10−16cm2for both POPOP and perylene. The upper limit of pulse rise time, required to produce a dye vapor discharge with positive gain, is discussed in relation to the value of &sgr;d.