Abstract: An apparatus and method for mixing concurrently downwardly flowing fluids is disclosed. The apparatus comprises a tray extending horizontally across a cross-section of a mixing chamber and having an aperture in the middle. A cap having a pair of opposing openings covers the aperture. Fluids pass through the openings into a pan adjacent a downstream portion of the mixing chamber and spaced below the tray. A tube encircling the aperture is disposed between the cap and the pan. The tube has notches cut into the lower wall to allow the fluids to enter the pan. Radially spaced risers extending from the bottom of the pan provide fluid communication to a downstream portion of the mixing chamber. The risers have upper ends which are above the notches and spaced below the tray. The mixing apparatus can be used between two catalyst beds in a vertical, fixed-bed reactor, wherein a hot, mixed-phase effluent stream is quenched with a hydrogen-rich gas in hydrodesulfurization, hydrocracking, and like processes.