Measurements of the concentrations of free impurity interstitials in ferritic steels are of considerable importance in the evaluation of the strain-aging susceptibility of the steels. Traditionally, determination of interstitial concentrations has been carried out by measurement of the heights of the carbon and nitrogen Snoek peaks using internal friction techniques. However, in the case of steels containing significant concentrations of nitride or carbide formers in substitutional solid solution, pre-precipitation clustering of free impurity interstitials around the substitutional impurities leads to extraordinary Snoek peaks. Such is the case in steel repair welds containing 1.5% by weight manganese. Nevertheless, by studying both the Snoek spectrum and the interaction of the impurity interstitials with dislocations, the susceptibility to strain aging can be assessed.