Surface aggregation as a necessary condition for interfacially induced crystal nucleation is studied with a surface forces apparatus (SFA).(1-3) As a model system, we use the chloride salt of the rod-shaped dye bis(dimethylamino)hepatmethine (BDH+Cl-) in water.(4) Even when the bulk solution is 3 orders of magnitude below saturation concentration, in a molecularly thin gap we find a structural oscillatory interaction force, whose periodicity and viscosity indicate a supersaturated salt solution. The increased gap concentration can be rationalized by ion-pair formation of the large molecule. If only one salt monolayer is trapped between the surfaces, optical concentration measurements yield an area coverage corresponding to freely rotating rod-shaped cations. If the surface coverage increases, the oscillatory force is replaced by a purely repulsive force indicative of ion surface aggregation. These salt aggregates thicken during an experiment. Eventually, crystallization occurs at a surface separation of about 100 nm. If the bulk concentration is only 1 order of magnitude below saturation, salt crystallization occurs at the first approach of the surfaces, indicating pronounced aggregation on the isolated surfaces.