Ultrafast bimolecular reaction kinetics between photoexcited biphenyl and carbon tetrachloride, observed by femtosecond time-resolved fluorescence spectroscopy, is successfully interpreted by theories of diffusion-controlled reactions. The obtained decay kinetics is well explained by Smoluchowski's theory of diffusion-controlled reactions when the parameter R, distance between the reactants, is 0.39 nm. A modified kinetic theory by Collins and Kimball also explains the results satisfactorily, when R is 0.40 nm and the bimolecular reaction rate constant k(act) is 3.4 x 10(11) dm(3) mol(-1) s(-1). It is suggested that molecular motions in solution for a time period of a few picoseconds is described by diffusion.