Due to its mass and energy selectivity, mass-analyzed threshold ionization spectroscopy provides information on the energetic position of the dissociation threshold of van der Waals complex cations. In this work we report the first observation of a shift of the dissociation threshold of van der Waals complexes in high Rydberg states induced by an applied electric field of several hundred V/cm. The detection of a approximate to 70 cm(-1) shift is possible for fluorobenzene . Ar because of its high density of optically accessible vibrational states around the dissociation threshold at 568 cm(-1). This shift is explained by a field-induced coupling between high and low Rydberg states of different series.