A new experimental methodology was developed to study the transient mixing process of the high temperature how field behind the backstep with low temperature fluid injected from the lower surface. Temporal data show that the Row temperature at locations between 0.2 and 0.4 reattachment lengths, Xr, at 1-mm height dropped faster than other places. Temporal temperature difference across the wall plate performed quite different trends before and after 0.6 time the reattachment length. The temporal Nusselt number distribution over the entire wall plate reached its peak value about 5-10 s after the wall injection initiated and gradually converged to a steady-state value. (C) 2001 Published by Elsevier Science Ltd.