Time-resolved THz spectroscopy is employed to investigate the ultrafast electron dynamics in gaseous Ar, SF6, and an Ar-SF6 mixture following ionization by an intense femtosecond laser pulse. The THz response of the free electrons can be described by the Drude model and yields the temporal evolution of the electron density and collision rate. The free-electron decay in the Ar plasma is accelerated enormously by adding small amounts of the electron scavenger SF6. This observation is assigned to electron attachment to SF6 and allows us to infer an electronic temperature of about 20000 K. The electron collision rate reflects the dynamics of the ion density showing that anions and cations neutralize on a 10-ps time scale. (c) 2006 Elsevier B.V. All rights reserved.