The redn.-oxidn., extn., and sorption techniques were examd. to prep. HReO4, The 1-3-stage extn. of aq. 25g/l. NH4ReO4, neutral or H2SO4-acidified, with tributyl phosphate (TBP) followed by reextraction with hot water gave =90% Re yields. Sorption on a cation-exchange resin (Purolite's Worm C160) gave a max. NH4+ yield of 94.94%. Column breakthrough data were detd. for 20-g beds of several resins. Regeneration and washing of the bed were troublesome. In the sorption technique gave the purer perrhenic acid while in the extraction technique the water phase on reextn. can contain org. impurities and reextn. of Re was incomplete.