[en] The authors have developed a new method for determining the isotopic composition of lithium that makes use of the reactions ⁶Li(d,α)⁴He, ⁷Li(d,α)⁵He*(n)⁴He and ⁷Li(d,n)⁸Be*(α)⁴He. The α-particles from these reactions are observed with surface barrier detectors placed 135⁰ relative to the incident 1.37 MeV deuteron beam. The isotopic ratio is directly proportional to the ratio of α-particle yields. With detectors of 100 μm depletion depth and proper bias one can eliminate the products of various (d,p) reactions that would otherwise seriously interfere with counting the α-particles from ⁷Li. Elastically scattered deuterons are removed by 2.6 mg/cm² mylar absorbers. The lithium is extracted by conventional cation exchange techniques, and targets are prepared in the form of LiClO₄ deposited on a thick metal backing. The evaluation or minimization of errors resulting from effects of finite target thickness, finite angular acceptance of the detectors, count rate and contaminant elements is discussed. From measurements on a large number of calibration standards it is concluded that at the 1% level the errors result primarily from counting statistics. (Auth.)