Silicon nanowires are expected to have applications in transistors, sensors, resonators, solar cells and thermoelectric systems(1-5). Understanding the surface properties and dopant distribution will be critical for the fabrication of high-performance devices based on nanowires(6). At present, determination of the dopant concentration depends on a combination of experimental measurements of the mobility and threshold voltage 7, 8 in a nanowire field-effect transistor, a calculated value for the capacitance, and two assumptions-that the dopant distribution is uniform and that the surface (interface) charge density is known. These assumptions can be tested in planar devices with the capacitance-voltage technique(9). This technique has also been used to determine the mobility of nanowires(10-13), but it has not been used to measure surface properties and dopant distributions, despite their influence on the electronic properties of nanowires(14,15). Here, we measure the surface (interface) state density and the radial dopant profile of individual silicon nanowire field-effect transistors with the capacitance-voltage technique.