Normalized 2¼Cr-1Mo steel was isothermally oxidized at 973 K in air to develop an oxide scale of about 40μm thickness. The pre-oxidized specimens were then subjected to thermal cycling between 973 and 373 K in air for a maximum of 15 cycles by thermogravimetry (TG). The kinetics of oxidation were continuously followed to monitor the spalling behaviour of the scale during each cycle, and its influence on the subsequent oxidation. Cracking and spalling behaviour of the scale during thermal cycling was also assessed by monitoring the acoustic emission (AE) signals. Both TG and AE tests suggested that spallation occurred during each cooling cycle, but after a few cycles of thermal cycling spallations were substantial. Electron-optical examinations of the surfaces and the cross sections of the oxidized specimens suggested the formation of two types of oxide scales, morphologically different from each other and dissimilar in their cracking and spalling pattern.