The salinity minimum at about 1,900 m in the Bay of Biscay, which lies along a well‐defined line on a θ,S graph, is used to test the idea of large‐scale mixing along a neutral surface. Contours of temperature and salinity values along a neutral surface indicate that the source region responsible for the salinity minimum lies off the slopes of Newfoundland. This description gains support from the continuity of both the salinity minimum on a T,S curve across the mid‐Atlantic Ridge and the distribution of the oxygen saturation contours on the mid‐Atlantic Ridge.