Abstract. An experimental study is presented of the effect of an isothermal heat treatment (aging) upon the structural properties of three metastable phases in ZrNb and TiV alloys, viz. (hcp), (bcc) and , formed by quenching the alloys from 1273K. 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 ZrNb and TiV 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.