This paper is the first of a series dealing with the stress-induced ordering phenomenon in substitutional solid solutions, known as the "Zener relaxation effect". A brief review of the two proposed mechanisms for this phenomenon, viz. the pair reorientation theory of Zener and the directional short-range order theory of Le Claire and Lomer, is presented. It is then shown that the best test of any theory will come from a study of the magnitude of the relaxation strength and its dependence on various parameters. With the aid of the assumption that one can assign characteristic bond lengths to AA, AB and BB bonds in solid solutions, it is shown that the pair reorientation theory predicts a relaxation strength which varies as the square of the deviation from Vegard's law. To test this prediction, data for the relaxation strength, ΔG, of polycrystalline samples from nineteen alloy systems are compiled, together with appropriate information from lattice-parameter data. For solid solutions of the noble metals it is shown that there is no relation between ΔG and the size factor, but that there is a rough correlation between ΔG and the magnitude of the deviation from Vegard's law. © 1961.