High-resolution near-IR spectra of jet-cooled HF-DF and DF-HF isotopomers are presented and analyzed for fundamental excitation in the HF-stretching region (3870 cm(-1)-3960 cm(-1)) and DF-stretching region (2840 cm(-1)-2880 cm(-1)), based on direct absorption of tunable IR difference frequency radiation in a slit-jet supersonic expansion (10 K). Spectra are obtained for excitation of all four stretching modes, *HF-DF, HF-*DF, *DF-HF, and DF-*HF (* denotes the vibrationally excited subunit), which probe both the hydrogen/deuterium bond donor and acceptor moieties in the complex. Vibrational redshifts and predissociation broadening measurements are compared with full 6D quantum calculations on theoretical HF dimer potential surfaces, which exhibit trends in qualitatively good agreement with experiment. Each of the three DF-stretch-excited bands are fit to spectroscopic precision (Delta v less than or similar to 0.0001 cm(-1)) by conventional high-resolution rotational analyses, cm whereas each of the four corresponding HF-stretch-excited bands appear extensively perturbed (Delta v approximate to 0.01 cm(-1)). This H/D isotope effect is interpreted as vibrational state mixing of the HF-stretch-excited species with a dense manifold of combination band states built on DF-stretch excitation, and therefore reflects intermolecular energy flow in the complex. Such vibrational stale mixing is further corroborated by observation of "dark" state transitions that can be tentatively assigned to *HF-DF isomer interacting with the nearly isoenergetic DF-*HF isomer. This state mixing would correspond to a vibrationally promoted "isomerization" over the tunneling barrier, and yield a spectroscopic measure of the difference in hydrogen bond dissociation energies [Delta D-0 = 74.7(5) cm(-1)] for the HF-DF and DF-HF isotopomers.