The chemical species of silica dissolved in sodium chloride (NaCl) solution were identified by means of fast atom bombardment mass spectrometry (negative ion mode). The concentration of silica in 0.1 M NaCl solution is < 0.6 mmol dm(-3) (mM) and application to the identification of the silicate species at low concentrations such as in natural waters level is also possible. An apparent peak at m/z 95, which corresponds to SiO(OH)(3)(-) in 0.1 M NaCl solution, was not confirmed owing to the interference of the peaks corresponding to NaCl(2)(-); however, peaks for complexes such as Si(OH)(2)O(2)Na(-), Si(2)(OH)(5)O(2)(-), Si(2)(OH)(4)O(3)Na(-), Si(2)(OH)(3)O(4)Na(2)(-), Si(2)(OH)(2)O(5)Na(3)(-), Si(4)(OH)(7)O(5)(-), Si(4)(OH)(6)O(6)Na(-) and Si(4)(OH)(5)O(7)Na(2)(-) were detected. The existence of the trimer and its Na(+) complexes such as Si(3)(OH)(7)O(3)(-), Si(3)(OH)(6)O(4)Na(-) and Si(3)(OH)(5)O(5)Na(2)(-) was not clearly shown. These complexes can be confirmed not only in the form of the anion itself (e.g. Si(2)(OH)(5)O(2)(-)), but also in the form of some complexes with sodium ions, such as Si(2)(OH)(4)O(2)Na(-). Copyright 2000 John Wiley & Sons, Ltd.