Electrospray ionization of alkali halide solutions in the negative ion mode results in the formation of cluster ions of the general formula, (MX)(n)X. However, alkali iodides form triiodide anion, I-3, in high abundance in addition to cluster ions. Br-3 ions are observed in low abundance. Also, mixed tri-halide anions, (IY)-Y-2 , are observed in high abundance when a small amount (<1%) of KI is added to other alkali halide solutions. These results are explained by the uniquely different physical characteristics of lithium and the iodide ions compared with others in the series. (C) 2017 Elsevier B.V. All rights reserved.