A method for preparing halogenated alkanes and alkenes which comprises reacting by contacting, for 0.005 to 0.1 second at a temperature of 675.degree. C. to 850.degree. C., a halogenating agent of the formula CCl.sub.x Br.sub.4-x, wherein x is an integer from 0 to 4, with a compound of the formula R.sub.2 CH.sub.2, wherein each R is H, Cl or Br, with or without the presence of inert gases. The utilization of the above conditions prevents the formation of large amounts of undesirable tars.