A small UHV chamber that fits in the sample compartment of a commercial Fourier transform infrared spectrometer (IBM‐98, Brucker) has been designed and constructed to acquire reflection/absorption spectra of species adsorbed and/or condensed on metal films vapor depositedinsituon the surface of a quartz‐crystal microbalance (QCM). The QCM is mounted on a variable‐temperature Au‐plated Cu block, which houses a second (reference) QCM, attached in turn to a liquid‐He cold finger. The unique capabilities of this apparatus make it possible to correlate quantitatively various spectral characteristics, including peak widths and intensities, with the absolute mass of adsorbed and/or condensed material per unit aream, or coverage, over a wide range ofmand temperatures. Results obtained using tetrahydrofuran (THF) condensed on freshly vapor deposited gold films have shown that the sensitivity of the instrument is sufficient to detectmTHFon the order of a single monolayer. Plots of the intensity of the most prominent THF peak at 1064 cm−1as a function ofmTHFwere found to deviate significantly from linearity in the range 0.1<mTHF<2.8 &mgr;g/cm2. In addition, the relative intensities of the most prominent THF peaks for even the largestmTHFexamined differed markedly from those of bulk‐like THF, an effect that can be attributed to a preferential orientation of THF microcrystals with respect to the underlying substrate. In the case of coverages on the order of one monolayer, such spectral behavior may arise from specific adsorbate/substrate interactions, which can cause a lowering in the molecular symmetry and/or distortions in the structure of the adsorbate. Evidence in support of a Au‐induced THF activation was obtained from independent temperature programmed desorption (TPD) measurements involving perdeuterated THF (TDF) performed in a separate UHV system. In particular, for exposures of about 1.5 L, the TPD spectra for the largestm/efeatures, i.e., 80, 48, and 30, revealed the presence of a peak centered at a higher temperature (∼180 K) than that found for much larger exposures (120 K).