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The effect of the separate additions of 1 at % of 13 different substitutional solute atoms on the internal friction spectrum of niobium containing oxygen and nitrogen as interstitial solutes has been studied at a frequency of 1 Hz. The substitutional solute atoms interact elastically and chemically with the interstitial solute atoms forming s-i complexes. The effect of the s-i interactions is to reduce the oxygen and nitrogen Snoek peaks (sometimes to zero) and to introduce into the spectrum new subsidiary peaks caused by relaxations of the s-i complexes.The substitutional solute atoms have been classified into three groups according to their effects on the internal friction spectrum. This classification can be justified in terms of the following factors: size factor, modulus factor, chemical affinity, and first ionization potential. It has been found that the s-i interactions within each group of the solute atoms are affected by the size factor, modulus factor, chemical affinity and the first ionization potential, in general, in this order of importance.

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