2¼Cr-1Mo steel has been widely used for high-temperature, high-pressure hydrogenation reactors such as hydrodesulfurizing reactors. Material degradations that have been found to occur during the service life of such equipment are hydrogen damage, creep embrittlement, and temper embrittlement. A brief review is made of these embrittlements, among which the latter has been considered the most serious. A study of the effect of chemical composition on temper embrittlement showed that susceptibility of 2¼Cr-1Mo steel to temper embrittlement can be well expressed by the J-factor [(Si + Mn) (P + Sn) × 10⁴] and therefore can be reduced by lowering the J-factor value. 2¼Cr-1Mo steel with very low J-factor is easily obtained by lowering silicon content by applying the vacuum carbon deoxidation (VCD) process. Temper embrittlement susceptibility of the weld heat affected zone can be also minimized by the same compositional countermeasures as in the base metal. Temper embrittled steel was found to be further deteriorated by the presence of hydrogen so as to exhibit extremely low fracture toughness. Such hydrogen embrittlement can be reduced by minimizing temper embrittlement susceptibility. Seeking the scale merit of units and to meet the future requirements of coal liquefaction processes, reactors of considerable size (much larger than now exist) are being considered. From this viewpoint, availability of material for heavy-section pressure vessels is discussed based on the heat-treatment characteristics of 2¼Cr-1Mo steel. The formation of polygonal ferrite must be avoided in order to ensure adequate mechanical properties of 2¼Cr-1Mo steel. This was found possible for a thickness up to about 450 mm when accelerated cooling in water was applied. Mechanical properties of a heavy-section, low-silicon 2¼Cr-1Mo steel with thicknesses of 450 and 300 mm were also studied, and sufficient and homogeneous properties were confirmed.