A number of trace elements and some intentional steelmaking additions are known to contribute to the temper embrittlement of susceptible steels. Most of the work done to date has been on individual elements in tempered martensitic steels. This work is concerned with steels in a tempered bainitic condition whose heat treatment and final structure simulate closely that of a large forging, and is designed to study some solute element interactions that were thought to exist. The effects of manganese and molybdenum on embrittlement by phosphorus are investigated as well as that of manganese in combination with silicon. A series of 18 heats of 3.5 nickel, 1.75 chromium, 0.20 carbon steel were made by vacuum-induction melting using high purity components. The levels of phosphorus, manganese, silicon, and molybdenum were varied to measure their influence on temper embrittlement. Embrittlement was measured as a shift in the transition temperature determined from the conventional V-notch Charpy impact tests. Both a step-cooling heat treatment and isothermal exposures at 750, 850, and 950 F were used as embrittling treatments. Isothermal exposures were run to 3500 h with embrittlement measured at 100, 1000, and 3500-h exposure. It was found that manganese contributes to temper embrittlement and magnifies or interacts with the effect of phosphorus. Molybdenum varying between 0.2 and 0.84 per cent is demonstrated to curtail embrittlement by phosphorus and not contribute to temper embrittlement itself. Silicon alone or with manganese is shown to contribute to temper embrittlement both during isothermal exposures and slow cooling through the critical temperature range. Limits on solutes may be placed so that a Ni-Cr-Mo-V steel will show no significant temper embrittlement during slow cooling or subsequent isothermal exposure.