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The phase relations in hydrous and anhydrous mid-ocean ridge basalt were determined at pressures of 18–28GPa. Liquidus phase relations in hydrous and anhydrous MORB are different. Garnet is the liquidus phase at pressures below 21GPa, Ca–Al (CAS) phase and stishovite are the liquidus phases at pressures of 22–27GPa, and stishovite and Ca-perovskite are the liquidus phases above 27GPa, whereas Ca-perovskite is a liquidus phase of anhydrous MORB at pressures above 23GPa. Under subsolidus conditions, we have found that in the hydrous MORB system the stability fields of Al-bearing perovskite and Na–Al (NAL) phase might shift to lower pressure by about 1.5GPa compared to the dry MORB system. This shift could be explained by oxidation of a garnet-bearing assemblage by hydrous fluid and formation of Fe³⁺-bearing aluminous perovskite at lower pressures relative to the anhydrous system …