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Abstract Abundance measurements for the volatile element phosphorus are important for measuring the metallicity in interstellar and circumgalactic gas, where their accuracies are limited by uncertainties in the oscillator strengths. We report updated oscillator strength values for two resonant transitions of the dominant ion PII, the transitions at 961.041 and 963.801 Å, which have historically shown large uncertainties. Using a combination of observational measurements and highly accurate quasi-relativistic Hartree–Fock theoretical calculations, we present an updated oscillator strength off= 0.147 ± 0.021 for the poorly constrained PIIresonant transition at 961.401 Å, which arises from the ground electronic state 3s²3p²³P₀to the excited level 3s²3p3d³D ${}_1^{\mathrm{o}}$. This result utilizes archival optical spectra obtained with the Very Large Telescope for the quasar PKS 0528–250, which has a damped Lyαabsorber atz= 2.811. We calculate a theoreticalf-value = 0.153 for 961.401 Å consistent with our empirically derived value, and calculate a theoreticalf-value = 1.79 for 963.801 Å. We also present theoretical oscillator strengths for the PIIresonant transitions at 972.779, 1124.945, 1152.818, 1301.874, and 1532.533 Å, as well as for multiple PIIfine-structure and excited-level transitions. The updatedf-value for the PII961 Å transition will be useful in future studies of P abundances, especially in sight lines where the 1152 Å line is saturated.