The “vibration–rotation” and “pure rotation” spectra of HF and DF in rare-gas matrices near 4°K have been studied, and assignments of the features due to the monomeric molecules have been made. The perturbations of the purely vibrational levels have been used to compute Lennard-Jones parameters for the interaction between the polar molecule and its neighboring atoms. The center of action of the dispersion forces has been calculated for HF, and the rotational perturbations have been evaluated. The spectral absorption frequencies are rather well reproduced. It has been found that the experimental frequency shifts are not consistent with a theory of rotational–translational coupling but that they, and other “anomalous” spectral features, are predicted by considerations of the anisotropies in the rotational potential function.