For application to heavy-thickness plates, steels based on the 2¼Cr-1Mo system should have sufficient hardenability to provide the appropriate microstructure and mechanical properties. This investigation was undertaken to evaluate the effects of several composition variations on the hardenability of 2¼Cr-1Mo steel, and thereby to provide useful information in applying this steel in sections up to 400 mm thick. Continuous cooling transformation diagrams were determined by dilatometry for variations of carbon, manganese, and minor additions such as nickel and vanadium. The critical cooling rate for polygonal ferrite formation, K_f, defined as the slowest rate at which the material can be cooled and still result in a microstructure free of polygonal ferrite, is considered the parameter of greatest significance. In terms of the average cooling rate between 800 and 500°C, K_f for 2¼Cr-1Mo steels containing 0.11 to 0.13 percent carbon is approximately 30°C/min; this corresponds to the cooling rate at the center of a 200-mm-thick plate during water quenching. Variations in carbon and manganese content and in the content of minor elements such as nickel significantly affect K_f. Vanadium does not significantly increase hardenability (decrease K_f), but it does increase temper resistance and hence the final strength after tempering and stress relieving. The average cooling rate (800 to 500°C) at the center of a 400-mm-thick plate during water quenching is about 8°C/min. A 2¼Cr-1Mo steel on the high side of the ASTM A 387 Grade 22 composition range and containing about 0.5 percent nickel has a fully bainitic microstructure after cooling at this rate. Maximum hardenability was observed in an experimental steel containing 0.15 percent carbon, 0.60 percent manganese, 2.37 percent chromium, 1.10 percent molybdenum, 0.50 percent nickel, and 0.08 percent vanadium that exhibited no polygonal ferrite after cooling at about 4°C/min. After tempering at 700°C for 120 h, this steel had a tensile strength of 590 MPa.