We present the first static measurements of the density of metallic liquids in the Fe-S system in the pressure and temperature range 1.5 GPa-6.2 GPa and 1500 K - 1780 K. Density is inferred from X-ray absorption experiments carried out with a large volume press at the European Synchrotron Radiation Facility. It is shown that increasing the amount of sulfur in liquid iron decreases the bulk incompressibility by -2.5 GPa per 1 weight% of S. These data are important for constraining the presence and amount of sulfur in the cores of small planetary bodies.