Thermalization of high-energy electrons in gaseous Ar at room temperature has been investigated by analyzing the imaginary component of the transient microwave conductivity produced by pulse radiolysis. The conductivity signal amplitude showing a peak due to the Ramsauer minimum has been correlated with the amplitude derived from calculations of the effective collision frequency using Margenau's formula assuming Maxwellian velocity distribution of electrons. Two approaches, using the peak and the plateau of the signal, for absolute normalization of the conductivity amplitude give results consistent with each other. It has been found that the excess mean electron energy drops very rapidly to about 0.2 eV and then decreases exponentially with a constant relaxation time. The thermalization time for 1 eV electrons to relax to 10% thermal energy has been determined to be 5.8 ms at 1 Torr Ar. Electron energy loss rate coefficients have been derived as a function of the mean electron energy.