Abstract The time dependence of the electron density during the afterglow period of plasmas produced in hydrogen was measured by means of the microwave cavity method. This time dependence was found to be exponential during the later part of the afterglow period for pressures smaller than 4.5 torr. The relevant time constant depended on the plasma excitation conditions. The mobility value 13.3 ± 0.5 cm 2/volt second calculated from the measured ambipolar diffusion coefficient is believed to refer to H + 3 ions moving in molecular hydrogen and is in good agreement with the values obtained from time-of-flight studies. The value 16.4 cm 2/volt second obtained with long plasma excitation pulse lengths most probability refers to H + ions moving in a mixture of atomic and molecular hydrogen. The studies indicated that one of the hydrogen ions, most probably H + 3, might have a recombination coefficient equal to or larger than 10 -7cm 3s -1.