An experimental study is presented of the effect of an isothermal heat treatment ('aging') upon the structural properties of three metastable phases in Zr-Nb and Ti-V alloys, viz. ! (hcp), # (bcc) and &#81, formed by quenching the alloys from 1273 K. Using neutron-diffraction experiments, the constitution of the aged alloys and the structural parameters of the resulting aged phases were determined. By combining lattice-parameter measurements with previously established correlations in the Zr-Nb and Ti-V systems, new information on the composition of the aged phases was obtained. For long aging times the ! and # phases seem to be approaching equilibrium conditions, which opens up the possibility of using quenching-and-aging experiments to gain insight into the metastable phase diagram of these systems, which is not accurately known from experiments.