A cryoscopic method has been used for the determination of number-average molecular weight of humic substances. Samples were dissolved in dilute alkali solution and then the pH of the sample solution was adjusted to that of the equivalent point of neutralization with the aid of canon exchange resin. Apparent molecular weight was corrected by determining the concentration of Na⁺ in the sample solution by means of the sodium ion electrode. The accuracy of this method is about 1-3%. For water soluble humic substance, it gave identical value as that obtained by Hansen and Schnitzers method. The Mn of humic substances from different origins examined by this method ranged from 600 to 3600.