PURPOSE: To efficiently obtain a βhalogeno-tert.alkyl isocyanate useful as an intermediate of a rubber additive, a herbicide, etc., by allowing a specified tert.-alkyl isocyanate to react with chlorine as a simple substance.

CONSTITUTION: A tert.-alkyl isocyanate of formula I [where R1 is an alkyl or a halogenoalkyl and R2 is an alkyl, a halogenoalkyl or a (halogen- and/or trifluoromethyl-substd.) phenyl] is allowed to react with chlorine as a simple substance, optionally under irradiation with a water-cooled high pressure mercury vapor lamp or in the presence of a catalyst, e.g. a peroxide such as cumyl peroxide or an azo compd. such as azoisobutyronitrile, in a suitable apparatus to obtain the objective β-halogenotert.-alkyl isocyanate of formula II (where X is Cl and Y is H or Cl).