The influence of solution composition (nature of anions and cations, pH) in the aqueous phase catalytic hydrogenation of C=C bonds (maleic acid) has been investigated.In weakly acidic solutions (of pH 3 and 6) the influence of the commonly used anions (ClO4-, HSO4-/SO42-, Cl-) is less important than that obtained in solutions of pH 0.3, found in previous works [Electrochim. Acta 45 (2000) 4299]. This difference can be ascribed to a weaker adsorption of the anion on platinum, as well as, to the different nature of the competitively adsorbed maleic acid species (molecular form or anionic adsorption) depending on the solution pH. In the presence of (hydrogeno)phosphates the lowest activities are obtained implying that these anions are the most strongly adsorbed on platinum.An important promotion of platinum activity in the presence of cations has also been found. This effect depends on the size of the cation and can be attributed to the modification of the work function and, consequently, of the adsorption properties of platinum. (C) 2003 Elsevier Ltd. All rights reserved.