Diazomethane is an extremely hazardous and unstable organic compound, (CH2)N=N=O, with a methane substituted diazo group, that is highly explosive when heated, shocked or contaminated, and used as a powerful methylating agent in chemical syntheses, but its production and handling require special expertise and equipment due to the high risks involved.

Addition of methyl groups. In histo-chemistry methylation is used to esterify carboxyl groups and remove sulfate groups by treating tissue sections with hot methanol in the presence of hydrochloric acid. (From Stedman, 25th ed)