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We report observations of the 13.5 $\mu m$ fundamental band of SiS in the spectrum of the heavily obscured carbon stars IRC +10216 and CIT 6. We have carried out a detailed model of the observed line profiles in IRC +10216, the brighter of the two sources. The lines in this source are formed in the inner region of the circumstellar envelope where the gas is accelerating and where the temperature ranges from 500-800ĘK. Our observations of IRC +10216 are best fit by a gradient in the abundance of SiS; we derive an abundance relative to molecular hydrogen of x(SiS) = 4.3 $x 10^{-6}$ at a distance of twelve stellar radii from the central star rising to x(SiS) = 4.3 $x 10^{-5}$ a few stellar radii from the star's surface. We interpret this gradient in terms of a depletion of Si onto grains in the acceleration region of the circumstellar envelope.
The relative line strengths in CIT 6 are quite similar to those in IRC +10216. Although the data on this source are of lesser quality, they are
consistent with an abundance level and distribution similar to that found in IRC +10216.
