In this paper, it is verified that the difference in mixing state between both lower and upper zones of a bubble column exists even in the shallowest bed with a clear liquid height L-0 equal to 0.56 m, From the time-dependent QM profiles, the following conclusions are obtained: The tracer concentration is relatively uniform in the Lower and upper zones, but the mixing properties of both zones are different, except for the earliest and latest stages of the mixing process, This is valid in the range of superficial gas velocities from 0.0178 mis to 0.0305 mis, provided that the clear liquid height is kept constant at 0.56 m, The mixing state of one zone is more advanced than that of the other zone when the superficial gas velocity is set as high as 0.0381 mis, except for the earliest and latest stages of the mixing process. This tendency is independent of column height. It is also proven that the operating time at which the concentration homogeneity is attained throughout the column is predictable and equal to the mass transfer (saturation) time tau (exp)(MT). For that purpose, Miller's (1974) method should be employed to correct the k(L) value derived by Higbie (1935). The authors propose the characteristic time tau (exp)(MT) to be adopted as a routine column shut-down time.