The clay mineralogy of sediments collected from the San Simón inlet, the Ría de Vigo, the Galician Platform, the Galician Margin and the Celtic Sea has been studied using XRD, IR and TEM with a microanalytical attachment. All samples consisted largely of dioctahedral mica and kaolin minerals, in addition to significant amounts of gibbsite, chloritic and smectitic minerals. The clay mineralogy of the sediments is generally consistent with terrigenous inputs from soils and weathered rocks of the Galician granitic hinterland. It is of particular interest that gibbsite, which is not a common constituent of soils of temperate climates, has previously been shown to occur in weathering profiles in this region and may therefore be regarded as an indicator of “continentality”, as suggested by Macías Vásquez and co-workers. The smectitic mineral is Fe-rich and also contains significant amounts of K. This mineral is likely to be ultimately of an authigenic origin and may possibly be important as a precursor mineral in a diffuse, non-granular glauconitization process.