Abrupt temperature inversions of as much as 0·74°C in 15 m are reported from stations above the crest of the Mid-Atlantic Ridge to the north of the Azores. These inversions occur below 700 m and mark the upper boundary of a saline layer of Mediterranean Water. By assuming that this layer has isentropically penetrated into a less-saline water column, the temperature-salinity diagrams of critical profiles are reconstructed. The thickest penetrations (150 m) were found over the median valley of the ridge. Comparable interfaces between cold less-saline water above and warm saline water below have previously been described by a (static) gravitational stability parameter. For the observed values of this parameter, tank experiments have indicated that there would be vigorous turbulent convection of both heat and salt across the interface. Evidence of the result of rapid diffusion of the core layer is found in a re-examination of the hydrographic data from this area. The conclusion is that the Mediterranean Water intermittently penetrates a continuously renewed, less-saline, water column above the crest of the Mid-Atlantic Ridge. Diffusion of the inversion layers then results in a step-discontinuity in the salinity of the core of Mediterranean Water across the North Atlantic Ocean.