Using in situ surface X-ray diffraction, the thermal stability of a self-assembled monolayer (SAM) of decanethiol, sandwiched between a Au(111) substrate and a van der Waals bound capping layer of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), is measured. While the bare SAM on Au(111) undergoes a melting transition at 100 degreesC, this transition temperature is raised to 115 degreesC if the SAM is capped by PTCDA. Possible mechanisms and implications of this capping-layer-mediated melting point enhancement are discussed.