Two different processes have been developed for the recovery of hydroxytyrosol (HT) and 3,4-dihydroxyphenyl glycol (DHPG) from olive oil mills waste. The antioxidant activity of several effluents has been characterised by four “in vitro” tests: antiradical capacity (ARC), ferric reducing power (P R) and inhibition of primary and secondary oxidation in lipid systems. HT-containing effluents exhibited higher or similar activity in ARC and P R than equimolecular quantities of vitamins E and C. In oxidative tests they were between both assayed vitamins. Except for commercial Hytolive®2, activities correlated with HT concentration of each sample. DHPG-containing effluent had higher activities than a solution with the same concentration of the synthetic DL-DHPG. This effluent was the most active in ARC and P R tests, its activity being similar to vitamin E in the inhibition of primary oxidation. Both phenolic compounds showed antagonist effect when they coeluted, except for P R assay. These results suggested that due to the low-cost procedure and to the antioxidant activity of effluents, they could be valuable as natural additives for food, pharmaceutical and cosmetic industries.