The crystallographic aspects associated with the formation of the hydride phase (fct) from the (hcp) phase and the(bcc) phase in Zr–Nb alloys have been studied in two distinct situations, viz., in the matrix in pure Zr and Zr–2.5Nb and in the matrix in stabilizedZr–20Nb alloy. The α-γ formation can be treated primarily as a simple shear on the basal plane involving a change in the stacking sequence. A possible mechanism for α–γ transformation has been presented in this paper. In this paper the→transformation has been considered in terms of the invariant plane strain theory (IPS) in order to predict the crystallographic features of the hydride formed. The lattice invariantshear (LIS)(11 0)_β[110]_β||(111)γ[121]γ has been considered and the crystallographic parameters associated with bcc→fct transformation, such as the habit plane and the magnitude of the LIS and the shape strain have been computed. The predictions made in the present analysis have been compared with experimentally observed habit planes. The/and/ interface has been examined by high resolution transmission electron microscopy (HRTEM) technique to compare with the interfaces observed in martensitic transformations