Five cross-strait hydrographic sections repeated several times during the Gibraltar Experiment in 1985--1986 are used to examine the structure of the interface layer between the inflowing Atlantic waters and outflowing Mediterranean waters in the Strait of Gibraltar. The interface is 60--100 m thick, with a strong vertical salinity gradient identified by fitting individual salinity profiles to a piecewise-linear, three-layer model. The interface is deeper, thicker, fresher, and colder on the west end of the strait than in the Narrows, where there is a minimum in thickness and a maximum in salinity gradient. Farther east, the interface thickens again and continues to get saltier, warmer, and shallower. Property variations in all three layers are also cast in terms of the three principal water types involved in the exchange. The traditional Knudsen model of exchange is extended to three layers, assuming that the interface is a transport-carrying third layer with uniform vertical shear. As much as half of the inflowing or outflowing transport occurs in the interface layer. Transport converges in both the upper and lower layers, implying, over the length of the strait, vertical exchange between layers that is comparable to about half the horizontal exchange. The richness of structure and complexity of interaction between the interface and the upper and lower layers argues against the use of two-layer models to characterize exchange through the Strait of Gibraltar. ¿ American Geophysical Union 1995