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A detailed analysis is presented of the absorption of energy from a thin-film dielectric waveguide by an absorbing medium that is brought into contact with one wall of the dielectric guide. It is shown that the exponential decay constant for the mode of the guide so obtained can closely approach that obtained for the free wave in the absorbing medium alone. The dependence of the attenuation on the guide parameters is examined and means for overcoming some of the problems imposed by the guidance process are discussed. It is concluded that the techniques can offer greatly enhanced sensitivity over the conventional internal-reflection spectroscopy and yet retain the attractive features of small sample volume and easy sample preparation. The technique also offers the possibility of constructing microscopic probes for the examination of spectra in otherwise inaccessible places.