Active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference. Hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways. These paths were absorption of three ultraviolet photons of frequency omega(1) and one vacuum ultraviolet photon of frequency omega 3 = 3 omega(1). The HI+ and I+ signals were modulated as the phase between the lasers was varied, with the HI+ signal lagging by 150 degrees +/- 15 degrees. A mechanism consisting of autoionization acid predissociation is proposed.