Summary: Sea-water from Deception Island was found to contain 0·55–1·48 mg Mn 1⁻¹ and 10·2-64·3 mg Si 1⁻¹. Reaction of Deception Island basalt and sea-water at 190 °C and 500 bars simulated the measured water composition but leaching experiments suggest that much of the Mn etc. may derive from local volcanic ash. Mn and Mg in thermal waters at Reykjanes have concentrations compatible with equilibrium with components of montmorillonite present as an alteration mineral of average composition Ca0·66Na0·33K0·03(Al0·28Fe0·35III Fe0·33IIMn0·01Mg1·28)[Si3·17Al0·83]O10(OH)2·2·55H2O. However, Fe is controlled by equilibrium with sulphides. Flux calculations based on these and other data place upper limits on hydrothermal Mn input to the oceans of 5–36×10¹¹ g yr⁻¹ and Mn accumulation in metalliferous sediments of 2–9×10¹¹ g yr⁻¹.