Mass spectrum of negative ions of trichloroacetic acid (TCA) in aqueous solution was obtained by mass spectrographic method of electrolyte solution electrospray in vacuum (ESESv). The TCA reactivity was neutralized by potassium hydroxide. The mass spectrum of TCA consists of the acid residue ions, (CCl3COO)-, hydrated by different number of water molecules. The (CCl3COO)- ion hydrated by two water molecules has the maximum intensity. The ESESv method can be helpful for determination of the composition electrolyte solutions in ecological studies. (C) 2013 Elsevier B.V. All rights reserved.