I propose to use the ISO SWS to obtain pairs of spectra of variable circumstellar silicate dust shells. I will use each pair of spectra along with radiative equilibrium dust shell models to derive the unique combination of dust shell structures (stellar temperature, ratio of inner shell radius to stellar radius, and optical thickness at a standard wavelength) and dust opacity spectrum which can match both the brighter and fainter of the pair of spectra of the shell. I request enough telescope time to obtain pairs of spectra for three different circumstellar dust shells. By comparing the resulting opacity spectra, I can investigate suggestions that there is a systematic change in particle opacity spectrum from thin to thick circumstellar shells.