We report on axial pn-junctions in GaAs nanowires. The nanowires were grown by MOVPE on (1 1 1)B GaAs substrates using the vapor-liquid-solid mechanism in combination with Au seed particles. At the low growth temperature of 400 degrees C any additional growth on the nanowire sidewalls can be excluded such that a pure axial pn-junction is realized, p-Type doping was provided by diethyl zinc, while tetraethyl tin was introduced for n-type doping. The impact of dopant supply was investigated both on structural properties and on carrier density. The carrier type was independently verified by processed nanowire metal-insulator FETs. The lengths of the whole pn-GaAs nanowires reach up to 20 mu m while their diameters are up to a few 100 nm, as defined by the Au seed particles used. The pn-GaAs nanowire device exhibits diode-like I-V characteristics and strong electroluminescence. While the reverse current is in the low pA-regime, the forward current reaches a few mu A, limited by the n-doped side. A diffusion voltage V-D=1.4 V is determined, which corresponds to the GaAs band gap energy. To our knowledge this is the first axial GaAs pn-diode realized in a single GaAs nanowire. (C) 2010 Elsevier B.V. All rights reserved.