Hafnium control rods removed at three intervals during the operation of the first core of the Shippingport PWR reactor were destructively tested for Hf isotope concentrations, corrosion, tensile properties, and fatigue behavior. The material examined had received a fluence ranging from 1 to 14 × 1020 n/cm2 thermal (E < 0.625 eV) and 0.9 to 6 × 1021 n/cm2 fast (> 1 MeV). Chemical analysis shows only minor irradiation-induced changes in the relative amounts of Hf isotopes. The surface buildup of transmutation products Ta and Lu, which may alter the out-of-reactor corrosion of Hf, apparently has no effect on in-service corrosion. Irradiation to a fluence of 6 × 1021 n/cm2 (> 1 MeV) produced a marked increase in strength and a modest decrease in ductility with increasing fluence, e.g., the yield strength at room temperature increased from 27 000 to 97 000 psi while the total elongation decreased from 35 to 12% and the reduction in area decreased from 35 to 20%. Similar relative changes occurred at 300 and 600°F test temperature. Strain-controlled fatigue tests at 600°F indicate only slight improvement in fatigue life at low strains after irradiation but shorter life than nonirradiated material at high-strain levels. The results of all the post-irradiation tests performed clearly indicate the adequacy of hafnium as a long-life neutron absorber material in pressurized water reactors.