We present an investigation into femtosecond time-resolved vibrational sum frequency spectroscopy as a tool for studying ultrafast surface dynamics. It is demonstrated that the presence of experimentally observed new spectral features does not necessarily mean that a novel chemical species has appeared on the surface. In addition, we present a simple model for calculating the sum frequency intensity as a chemical reaction takes place on the surface via a transition state or reaction intermediate. These calculations show that a transition state can be observed and its lifetime can be determined. © 2001 American Chemical Society.