Halogenation qf diazomethane with less than molar amounts of t-butyl hypohalites at-100” gives halodiazomethanes in good yields. Thermolysis at-10 'and photol-vsis of chloro-and bromodiazomethane yield free halocarbenes with undergo insertion reactions and additions to olefins. Halocarbenes from halodiazomethanes are less reactive than methylene but more reactive than the chlorocarbenoid generated from methylene chloride by a-elimination. ...