  A 12C32S, 13C32S, 12C34S, and 12C33S J = 2 - 1 line survey has been made to
study interstellar 32S/34S and 34S/33S ratios from the galactic disk. The four
CS isotopomers were detected in 20 star forming regions with galactocentric
distances between 3 and 9 kpc. From a comparison of line velocities, the C33S J
= 2 - 1 rest frequency is about 250 kHz below the value given in the Lovas
(1992) catalog. Taking 12C/13C ratios from Wilson & Rood (1994) and assuming
equal 12C32S and 13C32S excitation temperatures and beam filling factors,
12C32S opacities are in the range 3 to 15; average 32S/34S and 34S/33S isotope
ratios are 24.4 +/- 5.0 and 6.27 +/- 1.01, respectively. While no systematic
variation in the 34S/33S isotope ratio is found, the 32S/34S ratio increases
with galactocentric distance when accounting for the 12C/13C gradient of the
galactic disk. A fit to the unweighted data yields 32S/34S = 3.3 +/- 0.5
(dGC/kpc) + 4.1 +/- 3.1 with a correlation coefficient of 0.84. Since the
interstellar sulfur (S) isotopes are synthesized by oxygen burning in massive
stars, consequences for nucleosynthesis and models of chemical evolution are
briefly discussed.
