A process for reclaiming spent selenium filter mass containing an inert material. The spent mass is treated with a hydrogen peroxide solution for leaching out selenium content from unspent active substance present in the filter mass to form selenious acid. The filter mass is treated with aqua regia solution to dissolve mercury selenide present in the mass. The aqua regia solution is separated from the mass and isolated. Suitably, the filter mass, which now contains inert carrier material, is transferred with the isolated selenious acid, to production of new selenium filter mass. After partial neutralization of the aqua regia solution, mercury is precipitated out for disposal. Before this, elemental selenium can be separated from the aqua regia solution by adjusting the pH level and used advantageously for production of new filter mass. Thusly, reclaimed selenium content and inert carrier material can be advantageously used for production of new selenium filters.