To improve the conversion efficiency of tandem-type III-V multijunction solar cells, the (NH4)(2)S-x-treatment method was used to passivate the surface of the window layer (AlInP) of the cell. The conversion efficiency of the (NH4)(2)S-x-treated multijunction solar cell was increased more than 4%. Using X-ray photoelectron spectroscopy and current-voltage measurements, the mechanism of the conversion efficiency improvement is attributed to the reduction of the surface states by sulfidation passivation. It is indicated that the (NH4)(2)S-x treatment is an effective method for improving the performance of III-V compound multijunction solar cells. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2990714] All rights reserved.