An attempt is made, on the basis to published literature, to assess the amount of phosphorus that might be present in the steels without making it susceptible to grain boundary embrittlement. Embrittlement occurs when the general resistance to plastic flow is comparable to the stress required to separate crystals at their boundaries. A criterion is developed that enables a simple assessment to be made of the tendency to embrittle as a function of yield strength and the fraction of grain boundary sites that are covered by phosphorus. The latter is also sensitive to chemical composition, since some elements such as carbon segregate preferentially to phosphorus, giving rise to site competition that can permit a greater tolerance to the impurity. Other solutes affect the embrittling potency by different mechanisms, not all of which are clear.