The zirconium 91 isotopic content of zirconium is reduced by forming a solution of a zirconium compound and a scavenger of 8-hydroxyquinoline or its derivatives, and irradiating the solution with light at a wavelength which excites the compound so that it reacts with the scavenger. Because the molecules containing the zirconium 91 isotope remain excited longer, they react disproportionately with the scavenger. The reaction product, which precipitates from the solution, is therefore enriched in the zirconium 91 isotope. Micelle-forming agents can be added to the solution to enhance the isotopic enrichment.