The fatigue crack propagation behavior of a series of thick-section [20.3 cm (8 in.)] 2¼Cr-1Mo steels has been investigated in environments of gaseous hydrogen and ambient temperature air over a wide range of growth rates from near-threshold levels (10−11 m/cycle) to 10−4 m/cycle, as part of a program to characterize potential materials for coal gasification pressure vessel applications. Normalized bainitic-ferritic microstructures (ASTM A 387, Class 2, Grade 22) have been compared with quenched and tempered fully bainitic (ASTM A 542, Class 3) and fully martensitic (ASTM A 542, Class 2) microstructures, representing a range of yield strengths from 290 to 769 MPa. Although growth rates above 10−9 m/cycle are largely unaffected by microstructure and strength level, at near-threshold growth rates there is a marked deterioration in crack propagation resistance with increasing strength which becomes accentuated for tests in hydrogen environments. Further, the influence of gaseous hydrogen on fatigue crack propagation rates is found to be particularly severe at stress intensities far below KIscc, the threshold for hydrogen-assisted growth under sustained loading. At near-threshold levels (“true corrosion fatigue” regime) the presence of hydrogen gas enhances growth rates by up to two orders of magnitude compared with air, without a significant change in fracture mechanism. At higher growth rates (“apparent stress-corrosion fatigue” regime), a second acceleration in growth rates (up to 20 times compared with air) due to hydrogen is observed above a critical Kmax-value, which is both sensitive to frequency and load ratio and is associated with a predominately intergranular fracture mode. The characteristics of these distinct regimes of hydrogen-assisted fatigue crack propagation in 2¼Cr-1Mo steels are discussed in the light of the potential use of these steels for coal conversion pressure vessel construction.