The reaction between ethyl iodide and silver nitrate in water at 25° was found to be strongly catalysed by silver halides and other silver salts and by charcoal, and only weakly catalysed, or not at all, by finely divided metals, silica, and barium sulphate. A number of solids, especially silver iodide, also increased the rates of the reactions between aqueous ethyl iodide and thallous, lead, mercurous, and mercuric nitrates. The effectiveness of the various catalysts can be largely explained in terms of a Langmuir–Hinshelwood reaction mechanism and Pearson's theory of soft and hard acids and bases.