The technique of velocity map imaging, an enhanced resolution variant of ion imaging, is shown to be a useful method for determining the dissociation energy of van der Waals complexes. The method is demonstrated by measuring the dissociation energy of p-difluorobenzene-Ar in the S-1 state. From the spectroscopic shift of the S-1 <-- S-0 transition and the change in the ionisation energy between the complex and free p-difluorobenzene, the dissociation energies in the ground state of the neutral and cationic complexes are determined. The values so determined are 339 +/- 4 and 369 +/- 4 cm(-1) for the S-0 and S-1 states of the neutral complex, respectively, and 576 +/- 4 cm(-1) for the cation ground (D-0) state.