The effects of alloying elements in Ti-29Nb-13Ta-4.6Zr (wt%) (TNTZ) alloys with low Young's modulus for biomedical applications on microstructural evolution during aging, in particular, at an aging temperature of 400 degrees C have been determined. The peak hardness is obtained by co-precipitation of alpha and omega phases. O addition stabilizes omega phases; as a result, formation of omega is enhanced with increasing the O content as an alloying element, resulting in prevention of the growth of the alpha phases due to soft impingement. Because of the stress caused by the omega to alpha transformation, the alpha phase often contains defects within its internal structure. Although Zr is known to be a neutral element within Ti, here we show that Zr acts as weak beta stabilizer. At the beta/alpha interface, Zr enrichment appears to be due to a solute drag mechanism. In addition, a slight increase in Zr composition in the beta/omega interface has also been detected using atom probe. Dispersion of omega phases and segregation of Zr to the beta/alpha interface lead to fine alpha phase precipitation, resulting in an increase in the hardness of the O-added TNTZ alloys.