A method is described of producing any desired concentration of the isotope of hydrogen, H2, by electrolysis. Starting with twenty liters of water from an old commercial electrolytic cell and electrolyzing in four stages until only one-half of a cubic centimeter of water remained, this water had the specific gravity 1.073. Having shown that no large accumulation of the heavy isotopes of oxygen occurs and assuming that the density varies linearly with the fraction of H2, 2/3 of the hydrogen in this water is H2. It is shown that a further reduction of volume to one-quarter will give 99 percent H2. In our electrolyses the percentage loss of H1 is to the percentage loss of H2 in the ratio of five to one. By two methods we have found 1 in 6500, as a provisional value for the concentration of H2 in ordinary water.