The pressure of the α–ω phase transition in zirconium is quantified as a function of interstitial content. The pressure increases with increasing interstitial oxygen content and for the high purity (Zr 0 ) material occurs at 7.1GPa. Increasing the oxygen content increases the number of octahedral sites occupied; this is postulated to increase the pressure for phase transformation. Deformation behavior and substructural evolution of as-annealed Zr 0 zirconium under quasi-static conditions is compared to its response following shock prestraining at 5.8 and 8GPa. The reload response of Zr 0 zirconium shock prestrained to 8GPa exhibits enhanced hardening, while specimens shocked to 5.8GPa do not when compared to quasi-static constitutive behavior.