Semiconductor nanowires show promise for many device applications(1-3), but controlled doping with electronic and magnetic impurities remains an important challenge(4-8). Limitations on dopant incorporation have been identified in nanocrystals(9), raising concerns about the prospects for doping nanostructures(9,10). Progress has been hindered by the lack of a method to quantify the dopant distribution in single nanostructures. Recently, we showed that atom probe tomography can be used to determine the composition of isolated nanowires(11,12). Here, we report the first direct measurements of dopant concentrations in arbitrary regions of individual nanowires. We find that differences in precursor decomposition rates between the liquid catalyst and solid nanowire surface give rise to a heavily doped shell surrounding an underdoped core. We also present a thermodynamic model that relates liquid and solid compositions to dopant fluxes.