The structural and electronic properties of In12X (X = C, Si, Ge, Sn) and their anions have been investigated using the density functional theory at BLYP and B3LYP levels. The equilibrium geometries of In12X (X = Si, Ge, and Sn) and their anions favor the pseudo D-5h structures, while those of In12C and its anion tend to form C-s ones. The calculated adiabatic electron affinities of In12Si and In12Ge are 2.01 and 2.04 eV, respectively, in good agreement with the available experimental data. The magnetic moments of the ground state In12X are all zero, indicating that they have closed shell electronic structures. Thus, indium behaves as a trivalent atom in the clusters. (C) 2009 Elsevier B. V. All rights reserved.