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We present high spectral resolution mid-IR observations of SiS towards theC-rich AGB star IRC+10216 carried out with the Texas Echelon-cross-EchelleSpectrograph mounted on the NASA Infrared Telescope Facility. We haveidentified 204 ro-vibrational lines of 28Si32S, 26 of 29Si32S, 20 of 28Si34S,and 15 of 30Si32S in the frequency range 720-790 cm-1. These lines belong tobands v=1-0, 2-1, 3-2, 4-3, and 5-4, and involve rotational levels withJlow<90. About 30 per cent of these lines are unblended or weakly blended andcan be partially or entirely fitted with a code developed to model the mid-IRemission of a spherically symmetric circumstellar envelope composed ofexpanding gas and dust. The observed lines trace the envelope at distances tothe star <35R* (~0.7 arcsec). The fits are compatible with an expansionvelocity of 1+2.5(r/R*-1) km/s between 1 and 5R*, 11 km/s between 5 and 20R*,and 14.5 km/s outwards. The derived abundance profile of 28Si32S with respectto H2 is 4.9e-6 between the stellar photosphere and 5R*, decreasing linearly to1.6e-6 at 20R* and to 1.3e-6 at 50R*. 28Si32S seems to be rotationally underLTE in the region of the envelope probed with our observations andvibrationally out of LTE in most of it. There is a red-shifted emission excessin the 28Si32S lines of band v=1-0 that cannot be found in the lines of bandsv=2-1, 3-2, 4-3, and 5-4. This excess could be explained by an enhancement ofthe vibrational temperature around 20R* behind the star. The derived isotopicratios 28Si/29Si, and 32S/34S are 17 and 14, compatible with previousestimates.

The abundance of 28Si32S, 29Si32S, 28Si34S, and 30Si32S in the inner layers of the envelope of IRC+10216