Abstract The microstructures of ASTM A335-P11 1Cr-0.5Mo steel specimens removed from tubes which operated in a crude oil reformer for 1.05 × 10 5 and 1.22 × 10 5 h at about 520°C were examined by transmission electron microscopy. The main structural elements, ferrite grains, post-pearlite grains and ferrite grain boundaries, were investigated. It is shown that ferrite grains after 1.05 × 10 5 h in service contain mainly needle-like Mo 2C precipitates and a very small amount of M 23C 6 carbides. The amount of M 23C 6 carbides in ferrite grains is significantly higher after 1.22 × 10 5 h is service. At ferrite grain boundaries after both service periods a large amount of M 7C 3 and M 23C 6 carbides was found. After 1.22 × 10 5 h in service, M 6C carbides were also observed at some ferrite grain boundaries. Some specimens were also normalized at 930°C after 1.05 × 10 5 h service and then tempered at 725°C. Transmission electron microscopy of heat-treated steel revealed small spherical precipitates in ferrite grains instead of needle-like molybdenum carbides and the majority of ferrite grain boundaries were free of precipitates. The results of microstructural investigations are discussed in terms of the eventual changes in creep properties.