The electron density-dependence of effective diffusion coefficient D
s for electrons in argon Maxwellianized plasmas has been verified experimentally by means of afterglow technique. A calibrated microwave resonant cavity was used for measuring the electron density. By appending a synchronous detector to a conventional circuit, the sensitivity for the electron density <n
e> averaged over the volume of the cavity increased by about three orders of magnitude. Temporal behavior of the electron density between 1016 and 2.5×10
11 m
-3 at pressures of 46.6 and 66.5Pa was measured. In order to compare the experimental with theoretical results, <n
e> was transformed into the axial dcnsity, by the decay rate of which D
s was determined. The values of D
s were compared with those obtained theoretically by several authors in the range of ratios between 4 and 500 of the characteristic diffusion length Λ of the discharge tube to the Debye length λ
D0 for electrons. It was found experimentally that diffusion mode changed from ambipolar diffusion to transition modes when Λ/λ
D0 became smaller than about 100.
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