Zr-2.5Nb alloy is used for the pressure tubes in CANDU® reactor fuel channels. Three phases (α, β, and ω) of zirconium are known to exist in Zr-2.5Nb alloy pressure tube materials. An as received pressure tube is composed mainly of α-phase and approximately 8% β-phase material. In the last step of production, the pressure tube is given a steam-autoclave treatment (i.e., thermal ageing at 400°C for 24 h). This treatment results in decomposing the β-phase into an Nb-enriched β-phase and a metastable ω-phase material. In this present experiment, as received pressure tube materials were modified by high-temperature annealing and thermal ageing treatments. This paper presents a neutron diffraction investigation into the phase compositions in the modified materials at room temperature using the C-2 high-resolution neutron powder diffractometer at the Canadian Neutron Beam Centre.