The pearlitic carbide formed during isothermal decomposition of austenite in a commercial 1Cr-0.5Mo steel has been studied using electron diffraction and energy dispersive X-ray spectroscopy. The pearlitic carbide was found to be composed of both M3C and M23C6, with the M23C6 carbide becoming more common as the transformation temperature increased. The M23C6 carbide was richer in molybdenum (2.5–6.3 at.-%) than the M3C carbide (<2.5 at.-%). The average manganese and chromium concentrations in the pearlitic M3C and M23C6 formed at a given transformation temperature were similar. The average manganese concentrations (∼ 2.5 at.-%) did not vary significantly with transformation temperature, but the chromium concentration increased from ∼ 9 at.-% at a transformation temperature of 690°C to ∼ 18at.-% at 730°C. This indicates that the initial manganese concentration in the pearlitic carbide of commercial products is unlikely to be influenced strongly by the cooling rate following austenitisation, but the chromium concentration may be more sensitive. The pearlitic carbide in exservice 1Cr-0.5Mo steels from two superheater outlet headers and a virgin 1Cr-0.5Mo steel has also been characterised to confirm that the pearlitic M23C6 carbide does occur, although not commonly, in steels subjected to commercial austenitising treatments.