Zonguldak bituminous coal and Mugla-Yatagan, Seyitomer and Bolu-Goynuk lignites were O-methylated with potassium hydroxide-dimethyl sulphate (KOH-DMS) and acetone-potassium carbonate-dimethyl sulphate (acetone-K2CO3-DMS) methods. Effects of O-methylation on the swelling ratio and on the tetrahydrofuran (THF) solubility were studied. Methoxyl contents of coals were found to be larger after O-methylation with acetone-K2CO3-DMS (a) while O-methylation with acetone-K2CO3-DMS (b) resulted in significantly lower methoxyl formation. Coals O-methylated with acetone-K2CO3-DMS (b), although showed lower methoxyl contents, have comparably larger solubility than those treated according to the other two methods. Yet, no strong correlation with the methoxyl contents of coals and their swelling and solubility can be claimed. It can be reported here that the swelling and solubility, although dependent on the methoxyl content, are greatly influenced by the structure of the coal.