Abstract The isothermal decomposition behavior of a series of β-stabilized Zr + 14 – 20 wt.% Nb alloys has been characterized employing X-ray diffraction, optical microscopy, transmission electron microscopy and microhardness measurements. Aging treatments performed at 825 K for less than 100 h precipitated α-zirconium in the β-zirconium matrix and resulted in a significant hardening of the alloys. Aging times of approximately 100 h led to the formation of the equilibrium decomposition products, β-niobium particles in an α-zirconium matrix, without any loss in strength. Longer aging times produced a general coarsening of the structure and a subsequent decline in hardness. A comparison of the results obtained in this investigation with the structures and properties exhibited by the β-stabilized titanium alloys indicates that the isothermal decomposition behavior of the two systems is very similar.