The excimer formation that takes place following electronic excitation of van der Waals dimers of fluorene and dibenzofuran has been investigated in real-time using a picosecond pump-probe photodissociation technique. For both dimers, excimer formation is found to occur on a approximate to 40 ps time scale. The result indicates that the broadening in the excitation spectrum of the dibenzofuran dimer is due to inhomogeneous sources.