A new device for the distribution of vapors across an electron beam for molecular diffraction studies is described. The efficiency of the new method has made it possible to obtain molecular diffraction patterns with an electron microscope without need of alterations.The vapors are ejected from a circular crest of a diffusion box immediately surrounding the beam to give a heavy concentration of vapors in contact with the beam while releasing a small amount of vapor. The method has been standardized with well known patterns of CCl4and CBr4.