The IET spectrum of benzoate anion (C6H5COO-) adsorbed on the oxide surface of an Al/AlOx/Pb tunnel junction has been extensively studied at temperatures of 0.3 K and below. The natural lineshape of vibrational modes, deduced from the experimental data by fitting to computer simulations, is found to be well described by a Gaussian profile of typically 1–2 meV (8–16 cm-1) FWHM. Possible contributions to the natural linewidth associated with the tunnel-junction environment are considered and discussed. The effects of superconductivity in the Pb electrode on observed lineshape (asymmetry and undershoot) have also been studied in detail. Simulations highlighting the influence of natural and instrumental contributions to observed superconducting lineshape are presented. It is demonstrated that significant undershoot will only be observed in cases where the natural linewidth and thermal broadening components are substantially less than the superconducting energy gap width. However, an equivalent amount of modulation broadening is shown to reduce the size of the undershoot to a lesser extent. These predictions are borne out experimentally in observations on lines of various natural widths in the benzoate spectrum.