The wetting of thin oxide films on silicon by water is affected by fixed charge near the outer surface of the oxide. The authors show that existing data can be analysed to give both the depth and concentration of these fixed charges. The precise values indicate depths of a few angstroms and concentrations of a few times 1013 cm-2. There are hints that the values depend on the temperature at which the oxide is grown. They conclude that the wetting angle offers a further method of measuring fixed charge.