The origin of the low Young's modulus of cold worked Ti-36Nb-2Ta-3Zr-xO mass% polycrystals with a body-centered cubic (beta-phase) structure, referred to as gum metal, was investigated with a focus on the roles of oxygen concentration, the electron-atom (e/a) ratio, and the cold working process. Analysis of the temperature dependence of the microstructures and elastic properties of single crystals at x = 0.09, 0.36, 0.51% O using transmission electron microscopy and an electromagnetic acoustic resonance method, respectively, revealed that the shear moduli c' and c(44) of the 0.36 and 0.51% O alloys softened upon cooling near room temperature (RT) and exhibited low values at RT. This was because suppression of the alpha '' martensitic transformation by oxygen addition led to retention of the low stability single beta-phase state at RT. The Hill approximation indicated that the low c' and c(44) values caused by softening gave rise to the low Young's modulus, which is common to some Ti-Nb-based alloys with an c/a ratio of similar to 4.24. Analysis of the microstructures and elastic properties of solution-treated and cold worked x = 0.06, 0.30, 0.47% O alloy polycrystals at RT revealed that the Young's modulus increased upon 90% cold working due to formation of the alpha '' martensite phase (0.09% O) and omega phase (0.09, 0.30, and 0.47% O) with a high elastic modulus in the beta-phase matrix. However, increasing the oxygen concentration suppresses the increase in Young's modulus because oxygen addition decreases the amount of alpha '' and omega phases formed while retaining the low stability beta phase. Therefore, cold working combined with oxygen addition produces a low Young's modulus compatible with high strength. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.