Chirp effects on three photon absorption yields in I-2 vapor are investigated using femtosecond pulses with center wavelengths in the region 550-600 nm. Enhancements of as much as a factor of 3 are observed for chirped pulses with respect to transform-limited, zero chirp pulses. Theoretical considerations and model calculations suggest that wave packet dynamics play an important role. (C) 1998 American Institute of Physics.