Ultrafast bimolecular reaction kinetics between S-1 trans-stilbene and carbon tetrachloride is monitored by sub-picosecond time-resolved visible absorption spectroscopy. The observed kinetics is well explained either by Smoluchowski's theory or by Collins-Kimball's theory of diffusion-controlled reactions. These results suggest that the molecular motion in solution for a few picosecond time period is well described by diffusion.