Aging the 18R martensitic phase in CuZnAl alloys changes the critical β ↔ 18R transformation temperatures, and the critical transformation stresses. This diffusional phenomenon is known as martensite stabilization. In this work, the role of vacancy concentration and thermal treatment temperatures on the stabilization kinetics is analyzed. It is found that temperature determines the stabilization time start, while the initial vacancy concentration controls the stabilization rate. A model considering the evolution of vacancies during the stabilization time permits qualitatively a description of the process.