Normalized 21/4Cr-1Mo steel has been tempered at 998 K for durations up to 50 h, and then oxidized in air at 773, 873 and 973 K for a maximum duration of 1000 H. The extent of the prior tempering treatment was found to influence the oxidation behaviour of the steel significantly. In general, the oxidation resistance of the steel decreased with increasing duration of prior tempering. However, a pronounced influence has been observed during oxidation at 973 K, when at the end of a 6 h exposure the specimens with prior tempering for 50 h were found to have a weight gain 2.5 times more than the specimen without prior tempering. From the results of the pre- and post-oxidation analyses of the oxide-alloy matrix interfaces by SEM/EDAX, the observed oxidation behaviour could be attributed to the degree of depletion of free (effective) chromium from the alloy by the precipitation of secondary phases of chromium compounds during tempering for different durations. The secondary precipitates in the specimen tempered for 50 h at 998 K can become enriched in chromium by one order of magnitude more than that in the specimen with no prior tempering. Such a drastic depletion of chromium from the matrix causes the formation of a less protective inner oxide layer during oxidation. Acoustic emission tests carried out to assess the mechanical stability of the scale showed that the 50 h tempered specimen suffered cracking after about 4 h oxidation at 973 K, which results in subsequent enhanced oxidation.