It is shown that Te and Pb, which segregate at the surface during the epitaxial growth of GaAs, respectively, decrease and increase the surface diffusion length. This indicates that under the generic term ''surfactant,'' there are two types of surface segregating species which have opposite effects on the surface diffusion. It is suggested that the key parameter which imposes the surfactant-induced modification of epitaxial growth kinetics is the reactivity of a given pair of surfactant and growing material. Following this approach, it is concluded that surfactants occupying interstitial surface sites (nonreacting surfactants) increase the surface diffusion length whereas surfactants in substitutional sites (reacting surfactants) decrease it.