Differences in the inductively coupled plasma mass spectra of 0.2 mol l-1 HCI and 0.2 mol l-1 HClO4 are reported. An enhancement was observed of the ion signal in the mass spectra of several oxychlorine species (ClO2+, HClO2+, ClO3+, HClO3+, HClO4+ and H2ClO4+) in a solution of HClO4 compared with the spectra of an equimolar solution of HCl. Spectra of Ca in HClO4 showed enhanced formation of CaClO+, CaClO2+, CaClO3+ and CaClO4+ compared with equimolar solutions of Ca in HCl. Similarly, solutions of Mg in HClO4 showed enhanced formation of MgClO2+, MgClO3+ and MgClO4+ compared with equimolar solutions of Mg in HCl. Other major rock-forming elements (Na, Al, K, Sc, Ti, Mn and Fe) did not produce enhanced signals from oxychlorine polyatomic ions.