  We report the observation of the pressure-induced high-spin to low-spin
transition in FeS using new high-pressure synchrotron x-ray emission
spectroscopy techniques. The transition is evidenced by the disappearance of
the low-energy satellite in the Fe K$\beta$ emission spectrum of FeS. Moreover,
the phase transition is reversible and closely related to the structural phase
transition from a manganese phosphide-like phase to a monoclinic phase. The
study opens new opportunities for investigating the electronic properties of
materials under pressure.
