  Sulfur-bearing molecules play an important role in prebiotic chemistry and
planet habitability. They are also proposed probes of chemical ages, elemental
C/O ratio, and grain chemistry processing. Commonly detected in diverse
astrophysical objects, including the Solar System, their distribution and
chemistry remain, however, largely unknown in planet-forming disks. We present
CS ($2-1$) observations at $\sim0."3$ resolution performed within the ALMA-MAPS
Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296,
and MWC 480. CS is detected in all five disks, displaying a variety of radial
intensity profiles and spatial distributions across the sample, including
intriguing apparent azimuthal asymmetries. Transitions of C$_2$S and SO were
also serendipitously covered but only upper limits are found. For MWC 480, we
present complementary ALMA observations at $\sim0."5$, of CS, $^{13}$CS,
C$^{34}$S, H$_2$CS, OCS, and SO$_2$. We find a column density ratio
N(H$_{2}$CS)/N(CS)$\sim2/3$, suggesting that a substantial part of the sulfur
reservoir in disks is in organic form (i.e., C$_x$H$_y$S$_z$). Using
astrochemical disk modeling tuned to MWC 480, we demonstrate that
$N$(CS)/$N$(SO) is a promising probe for the elemental C/O ratio. The
comparison with the observations provides a super-solar C/O. We also find a
depleted gas-phase S/H ratio, suggesting either that part of the sulfur
reservoir is locked in solid phase or that it remains in an unidentified
gas-phase reservoir. This paper is part of the MAPS special issue of the
Astrophysical Journal Supplement.
