The excitation functions for ion-pair formation in supersonically cooled HF and DF have been measured using synchrotron radiation with photon resolutions of 0.12 and 0.06 nm. The excitation functions for both molecules are characterized by an intense, sharp peak, essentially coincident with the thermodynamic onset for ion-pair formation, followed by weaker, broader structure to higher energy. All of the structure is interpreted as arising from the photoexcitation of Rydberg states of the neutral molecules that are predissociated by the V (1) Sigma(+) ion-pair state potential. Ab initio calculations using multichannel quantum defect theory to include both predissociation and autoionization enabled assignments of the observed structure to be made and the construction of simulated ion-pair excitation function spectra in an energy region encompassing similar to 0.25 eV of the lowest energy part of the experimental excitation functions. The intense first peak in the ion-pair excitation function of both molecules is assigned to a high vibrational level of the 3s sigma Rydberg state converging to the A (2) Sigma(+) ion state, while most of the structure immediately following the first peak is assigned to d Rydberg complexes converging to the v(+) = 1 level of both X (II)-I-2 ion substates.