[en] The instant invention relates to a process for separating a material into two or more parts in each of which the abundance of the isotopes of a given element differ from the abundances of the isotopes of the same element in said material. More particularly, the invention relates to a method for the isotopically selective excitation of gas phase molecules by infrared radiation and the conversion of these molecules either by the same radiation or by excited state chemical reaction to form a product which may be separated by means known in the art. The instant invention is useful for but not limited to the separation of the principal isotopes of uranium