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Microwave measurements of electron density in the afterglow of a transient mercury-vapour plasma discharge, by the oblique-incidence method, indicate distinct responses prior to the main response. Such responses appear shortly before the central part of the column becomes transparent to the microwave probing signal and correspond to multiple reflections at the plasma boundary. The time constant of electron decay is deduced from the responses which appear at definite times, and it is shown that the principal electron-removal process is ambipolar diffusion from calculations based on an assumed electron-density profile. Extrapolation back to the time corresponding to peak current indicates an extremely high degree of ionisation.