A time-dependent multi-term solution of the Boltzmann equation is used
to calculate the drift and diffusion coefficients of electron swarms
in gases under the influence of a time varying electric field. Two mod
el gases are considered and for a.c. electric fields results are prese
nted for a wide range of applied frequencies. Of particular interest i
s the anomalous temporal behaviour of the longitudinal diffusion coeff
icient, which is discussed here for the first time.