An accurate theoretical study of ionization potentials and f values in the sodium iso-electronic sequence has been performed using a frozen core, multi-configuration Hartree-Fock procedure.The Z-dependence of core-polarization and its effect on the transition integral are investigated. The initial increase in both these quantities as Z increases is explained in terms of a relative collapse in the size of the atom. Analytic expressions are derived for 3s–3p, 3s–4p, 3p–4s, and 4s–4p transition integrals.Core-polarization has reduced the f values for the 3s–3p transitions somewhat, but still leaves a discrepancy of up to 25% between theory and recent beam–foil results.