The adsorption of a binary liquid mixtures onto a weakly attractive substrate has been studied in the vicinity of bulk liquid phase separation. A wetting transition between two different kinds of wetting films has been found. Below the wetting transition temperature, homogeneous films of one liquid phase wet the substrate. Above the wetting temperature, the wetting films contain both liquid phases. The surface phase transition line associated with this wetting transition is found to extend to both sides of the bulk phase separation line. On one side it is a prewetting line, and on the other side it becomes a line of triple point induced dewetting transitions. The experimental system used to study this behavior was liquid helium mixtures on a cesium substrate. (C) 1998 Elsevier Science B.V. All rights reserved.