Abstract: A process for manufacturing pure diosmin suitable as a therapeutic agent comprising acetylating hesperidin, observing by means of ultraviolet absorption the progress of the acetylation reaction and continuing such reaction until there disappears the band initially occurring at a wave length in the range of 264 to 280 nm and there appears in place thereof a new maximum at a wave length of about 330 nm. Thereafter the acetylation product is brominated, the obtained bromination product hydrolyzed and the obtained product after precipitation isolated, in order to obtain diosmin having a bromine content less than 0.1%.