The mechanical performance of metastable β Ti alloys can be markedly enhanced by adequate thermomechanical processing. A notable example is the Ti-5553 (Ti-5Al-5V-5Mo-3Cr-0.5Fe wt.%) alloy, the yield strength of which is over four-fold higher than that of CP Ti. An effective approach for the mechanical strengthening of metastable β Ti alloys involves the precipitation of the finely dispersed α phase into the β phase matrix via aging heat treatment at intermediary temperatures, usually between 500 and 650 ^oC. The precipitates hinder dislocation motion in the β phase, thereby enhancing the mechanical strength. Considerable effort has been devoted to elucidating the effect of the ω phase on α phase precipitation into the β matrix with the aim of improving the mechanical properties of metastable β Ti alloys. On the other hand, an alternative approach for enhancing the strength of these alloys involves the addition of microalloying elements. Ti-Cu alloys have been the subject of several studies in recent years, especially because Ti and Cu form an active eutectoid system, that is, its fast kinetics of phase transformation makes it challenging to hinder eutectoid transformation, even when extremely high cooling rates are applied for β quenching. Therefore, Cu is not an effective β stabilizing element and even in small additions, it may result in Ti₂Cu intermetallic compound precipitation. On the nanometer scale, Ti₂Cu precipitates can alter α phase precipitation in metastable β Ti alloys upon aging heat treatment and thereby modify their final mechanical behavior. The aim of this study was to examine the aging response of a β quenched Cu-modified Ti-5553 alloy, α phase precipitation therein and the resulting mechanical behavior. It was found that Cu addition resulted in the precipitation of a nanometric Ti₂Cu intermetallic compound, which upon aging, affected α phase precipitation and therefore improved mechanical strength at the expense of ductility.