The susceptiblity to temper embrittlement during heat treatment of several large steam turbine generator rotor steels was investigated. Tests were performed on laboratory heats and on material from test prolongations and center core trepans from a number of rotor forgings. Chemical compositions, tensile, and Charpy impact properties were determined and are included for a number of Ni-Mo-V, Cr-Mo-V, and Ni-Cr-Mo-V steels. The latter included some which were silicon deoxidized (silicon 0.15 to 0.35 percent) and others which were vacuum-carbon deoxidized (silicon 0.10 percent max) by vacuum degassing without addition of metallic deoxidizers. Charpy fracture appearance transition temperatures are given for the as-received condition, after deembrittlement by water quenching from 1100 F (594 C), and after embrittlement by slow step cooling from 1100 F (TTSC). The results of computer analyses of the relationship between properties, temper embrittlement, and chemical composition are presented and discussed. It was found that embrittlement susceptibility varies greatly with composition. These differences in susceptibility are associated with the presence of the known embrittling elements, phosphorus, silicon, arsenic, antimony, and tin. The balance of major elements such as nickel, manganese, molybdenum, and chromium also affects the degree of embrittlement. The presence of both nickel and chromium sharply increases the embrittling effect of phosphorus and perhaps of other elements as compared with compositions which contain only one or the other. Restrictions on manganese and molybdenum also appear beneficial. From as-received fracture appearance transition temperature and as-received embrittlement data it is apparent that temper embrittlement produced during heat treatment limits the fracture toughness attainable in massive sections of these steels and makes the control of temper embrittlement susceptibility of great value in components which must be heated within, or cooled slowly through, the range of embrittlement. Improved control of the residual elements to attain the low levels which appear desirable will require further advances in melting practice and selection of raw materials, in analytical procedures, testing, and specification of materials.