Ices containing (\mathrm{H}{2} \mathrm{O}) and small amounts of simple hydrocarbons, either in the form of clathrates or as intimate physical mixtures, are thought to be common outer Solar System condensates that were incorporated into comets and other bodies in the outer Solar System. We have previously shown that plasma irradiation of (\mathrm{H}{2} \mathrm{O}) frost containing small quantities of (\mathrm{CH}{4}, \mathrm{C}{2} \mathrm{H}{6}, \mathrm{C}{2} \mathrm{H}{6}+\mathrm{NH}{3}), or (\mathrm{C}{2} \mathrm{H}{2}) produces complex organic residues (tholins), noted that residues from (\mathrm{H}{2} \mathrm{O} / \mathrm{CH}{4}) and (\mathrm{H}{2} \mathrm{O} / \mathrm{C}{2} \mathrm{H}{6}) have very similar spectra (W. R. Thompson, B. G. J. P. T. Murray, B. N. Khare, and C. Sagan, 1987, J. Geophys. Res. 92, 14933-14947), and used these tholins to reproduce the 3.4- (\mu \mathrm{m}) spectra of cometary comae (C. Chyba and C. Sagan 1987, Nature 330, 350-353; C. F. Chyba, C. Sagan, and M. J. Mumma, 1989, Icarus 79, 362-381). Through 50 separate irradiations of (6:1) (\mathrm{H}{2} \mathrm{O} / \mathrm{C}{2} \mathrm{H}{6}) ice over a 5 -month period, we have now obtained enough tholin for the determination of its optical constants between 0.06 and (40 \mu \mathrm{m}). Thin films of tholin were vacuum-evaporated onto (\mathrm{CaF}_{2}) and (\mathrm{Csl}) substrates. The thin film spectrum exhibits all features present in the spectrum of the original residue except for those due to traces of adsorbed water. The imaginary part of the defractive index, (k), was obtained by transmission measurements on thin film samples and by Kramers-Kronig analysis on reflectance. The real part of the refractive index, (\boldsymbol{n}), was determined using ellipsometry and Kramers-Kronig analysis. The results may be useful in interpretation of cometary and outer Solar System spectrometry. 1993 Academic Press, inc.