The effect of pressure on the wetting behavior of liquid Fe-Ni-S alloy in a solid silicate matrix was investigated. Experiments were performed on a chondritic starting material in piston-cylinder and multi-anvil apparatuses over a pressure range of 2 to 20 GPa, at temperatures below the silicate solidus. The dihedral angle of the quenched liquid iron alloy in contact with the silicate matrix exhibited no change over the pressure interval. The average value of 108¿ for the dihedral angle is well above the 60¿ critical value for efficient percolation of iron alloy through a solid matrix, therefore core formation by percolation at upper mantle pressures is not feasible. We conclude that melting of the silicate mantle may be required for efficient planetary core formation. ¿ American Geophysical Union 1996