pn junctions formed with complex oxides may be of interest in emerging fields of electronics and energy conversion owing in part to their rich electronic structure. We report on the successful synthesis of epitaxial La1.875Sr0.125NiO4 (LSNO) films on Nb-doped SrTiO3 (Nb:STO) single crystals and a detailed study of their electrical properties. The junctions measured displayed highly rectifying current-voltage characteristic from 298 to 373 K. Mechanism for rectification is likely due to the formation of a pn junction between heavily doped n-type Nb: STO and semiconducting p-type LSNO. Capacitance-voltage (C-V) measurements as a function of temperature and frequency were used to determine the admittance of the pn junction. We investigate the thickness dependence of capacitance and highlight the role of defects in influencing the electronic properties of complex oxide pn junctions. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.004202jes] All rights reserved.