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 = $sqrt{K/rho}$), 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.