Atomic-absorption spectrophotometry has been used to determine trace amounts of metal ions in aqueous solution and to investigate the effect of pH upon their stability, solubility and adsorption. It was found that for the fifteen metal ions investigated, namely those of aluminium, calcium, cobalt, chromium, copper, iron, magnesium, manganese, molybdenum, nickel, lead, strontium, titanium, vanadium and zinc, a pH of 1·5 or less was necessary to ensure that all of these metal ions remained in solution at low levels of concentration.