The desorption and scattering of As-4 and As-2 species from the surface during the growth of germanium films on Si(100) with continuous As-4 deposition is monitored using laser ionization time-of-flight mass spectrometry. A significant increase in the flux of As-2 from the surface is observed when the Ge flux is admitted to the surface. Upon discontinuation of the germanium growth process, the As-4 and As-2 signal levels remain at this increased level. A comprehensive study of the desorption fluxes of As-4, As-2, and As species from both Ge(100) and Si(100) substrates was performed as a function of substrate temperature and incident As-4 flux to determine the kinetics of desorption of the different species from the substrates. The behavior of the As-2 desorbed fluxes as a function of surface temperature is qualitatively different on the Ge(100) and Si(100) substrates. The results indicate that the catalytic cracking of As-4 to As-2 is more effective on Ge(100) compared to Si(100) at substrate temperatures between 800 and 1000 K, most likely because of more rapid desorption of As-2 at a given temperature on Ge(100). A phenomenological activation energy for the desorption of As atoms from Ge(100) of 1.2 +/- 0.4 eV is also obtained. The implications for the surfactant enhanced growth of Ge on Si(100) are discussed.