Density of liquid Fe-Si alloys was measured in situ up to 5 GPa-1725 K by an X-ray absorption technique using synchrotron radiation. Increasing the amount of silicon in liquid iron decreases the bulk incompressibility by only 0.5 GPa per 1 weight% of Si. These data confirm our previous prediction of a negligible effect of Si on liquid Fe bulk properties, prediction based on the observation of a similar local structure in liquid Fe and liquid Fe-Si alloys. Si and S have therefore opposite effects on P-waves velocity (vP = ffiffiffiffiffiffiffiffi K=r p ), both elements reduce the bulk density of liquid iron but only S affects its compressibility. Since compression-wave velocities in the Earth's outer core are slightly higher than in pure liquid Fe in the same P-T conditions, it implies that Si would correct this discrepancy while S would increase it. INDEX TERMS: 1015 Geochemistry: Composition of the core; 3630 Mineralogy and Petrology: Experimental mineralogy and petrology; 3919 Mineral Physics: Equations of state; 3924 Mineral Physics: Highpressure behavior; 3954 Mineral Physics: X ray, neutron, and electron spectroscopy and diffraction. Citation: Sanloup, C., G. Fiquet, E. Gregoryanz, G. Morard, and M. Mezouar (2004), Effect of Si on liquid Fe compressibility: Implications for sound velocity in core materials