The α-β phase transformation behavior of Zr-2.5Nb (in mass%) has been characterized in real time during an in situ neutron diffraction experiment. The Zr-2.5Nb material in the current study consists, at room temperature, of α-Zr phase (hcp) and two β phases (bcc), a Nb rich β-Nb phase and retained, Zr rich, β-Zr(Nb) phase. It is suggested that this is related to a quench off the equilibrium solubility of Nb atoms in the Zr bcc unit cells. Vegard's law combined with thermal expansion is applied to calculate the composition of the β-phase, which is compared with the phase diagram, revealing the system's kinetic behavior for approaching equilibrium. In situ neutron diffraction has been used to characterize the phase transformation process of the nuclear structural material Zr-2.5Nb (mass%) while subjected to heating and cooling cycles. The shift of diffraction peaks, as shown in the image, has been evaluated by Vegard's law to track the change of Nb concentrations in the β-Zr(Nb) phase, which reveals the system kinetics for approaching equilibrium.