An investigation of a flow reversal that was observed to occur during narrow channel flow boiling is reported in this paper. The investigation was undertaken to determine the conditions under which the flow reversal occurred and to determine its cause. High-speed photography was used to study the sequence of events that occurred during each cycle of the flow reversal.Two-phase flow instability models involving boiling crisis were examined and correlations for the occurrence of boiling crisis based on the flow conditions and the test section geometry were tested with the experimental data. Strong agreement was found between the onset of the flow reversal and the prediction of CHF under subcooled flow boiling conditions as well as between the predicted rewetting times and the period of the flow reversal. As a result, the instability was deemed to be caused by the onset of CHF and to be the result of dryout and rewetting of the heated surface. (C) 2003 Elsevier Science Ltd. All rights reserved.