A new method of molecular weight measurement of liquids based on the use of the test sample as solvent in a G.L.C. column is described. The theory is outlined and two experimental approaches discussed. Experimental data for some liquids of molecular weights, approximately 300, 400 and 1200, are presented and the results are shown to be excellent. The method, which may be extended to higher molecular weights with some loss of accuracy, has the advantage of relative rapidity.