Dynamic aspects of methylation of catechol estrogen with diazomethane were investigated by means of thin-layer chromatography. The methylation rate of the hydroxyl group at the C-3 position was almost the same as that of the C-2 hydroxyl group in the reaction of 2-hydroxyestrogen, and 2-3 times that of the C-4 hydroxyl group in the reaction of 4-hydroxyestrogen. In these experiments, the maximum yields of 2-methoxyestrone, 2-hydroxyestrone 3-methyl ether, 4-methoxyestrone and 4-hydroxyestrone 3-methyl ether were 32, 39, 13 and 70%, respectively. In addition, demethylation of catechol estrogen dimethyl ethers with boron tribromide and synthesis of 4-hydroxyestrone monomethyl ethers are described.