The methylation of humic acids (HA) with dimethylsulfate in acetone and methanol followed by the iodometric determination of the methoxy groups (Zeisel reaction) were applied to determine the contents of -OH groups in solid samples of HA of different origins. For the coal- and peat-derived HA samples, the contents of -OH groups determined after methylation in acetone ranged from 6.6 to 8.7 mmol/g, whereas the contents of -OH groups determined after methylation in methanol ranged from 4.0 to 5.0 mmol/g. These differences may be related to the content of carboxylic groups in the HA molecule that were not methylated in methanol, as confirmed by a comparison with results of conventional titrimetric determinations. Observed differences were interpreted as results of different polarity of both solvents and alkalinity of the reaction mixture during the methylation. The contents of alcoholic groups as well as some other minor -OH groups can be estimated using the -OH group contents obtained after methylation in both solvents together with the results of the conventional determinations of acidic functional groups. A repeatability of the -OH groups determination as estimated from a series of triplicate analyses of different HA samples (n=7) was in range of 0.15-0.73 mmol/g and 0.08-1.06 mmol/g (standard deviations) for methylation in acetone and methanol, respectively. Thus, the average repeatability of the -OH groups determination was estimated to be 0.38 and 0.50 mmol/g for methylation in acetone and methanol, respectively.