Results are presented for a mass spectrometric study of reactions between alkali halide molecular beams and vaporizing single crystals of different alkali halides. Shuttering the beam yields a temperature dependent decay of reaction products from their steady state surface fluxes. We determined Arrhenius pre-exponentials and activation energies from the decay rates of unreacted beam diatomic (monomer), dimer products, and beam–crystal ion exchange products (monomers). The kinetic curves were primarily composed of a slow decay with a smaller amount of a faster decay. The pre-exponentials and activation energies exhibit a close correlation (lnAαEa) for energies ranging from 2–57 kcal/mole. This compensation effect has allowed our direct measurement of several processes in a narrow range of first order decay constants (0.02–4.0 sec−1). Analysis of the decay curves and extensive modeling of the crystal surface defects identify the measured activation barriers with specific surface processes. We have assigned dimer product and unreacted beam monomer activation energies to barriers against motion from ledge to surface desorption sites. Beam–crystal ion exchange reactions occur at kink defects and the rate determining step for monomer product formation is the dissociation of a dimer intermediate at the kink. Some decay measurements showed evidence for bulk diffusion of diatomic and atomic species.