The net proton surface charge density of silica suspended in water-acetone mixtures was studied by potentiometric titration. LiCl and NaCl were used as background electrolytes at concentrations of 10(-1), 10(-2), and 10(-3) mol L-1. The results showed that acetone lowers the net proton surface charge density of silica and that the greater the decrease, the greater the acetone concentration. The surface charge density of silica also is very sensitive to the nature of the background electrolyte, LiCl producing much lower surface charge densities than NaCl. The concept of free energy of transfer (Delta G(t)(0)) of an electrolyte between two different solvents was applied to explain the results in a qualitative manner.