When concrete is exposed to seawater, the composition of the outer most millimeters of the concrete will considerably alter compared to the composition of the bulk concrete.Seawater leads to a zoned attack: the outermost zone is generally enrichement in magnesium, followed by a sulfur enriched and finally a chlorine enriched zone.This paper focuses on the magnesium enriched zone which is relatively thin, in the order of 100 μm.The phase composition of this zone was examined using SEM-EDS on polished sections.The results from a selection of samples from marine structures in Norway and Denmark is presented.Magnesium seems to be present in an M-S-H like phase with a Mg/Si ratio ranging between 1.0 and 0.5 and an Al/Mg ratio of 0.05-0.3.The composition of the M-S-H phase observed in the field samples was compared to the M-S-H phase observed in cement paste submitted to an accelerated seawater exposure in a laboratory test.Their compositions were similar.