The reaction in perchloric acid solution between Hg2+ and Fe2+ was found to proceed extremely slowly until it suddenly exhibited an autocatalytic surge. In conjunction with this rise, a yellow-green by-product formed and disappeared again when the reaction came to an end. In the related reaction between Hg-2(2+) and Fe2+, the autocatalytic behaviour started early and the yellow-green by-product again appeared. No metallic mercury or other solid formed in either reaction, but the production of colloidal mercury was demonstrated by the Tyndall effect and by electrophoresis experiments. A five-step mechanism for these reactions is proposed and incorporated into a computer simulation called Mercury. The behaviour predicted by these simulations agrees well with the unusual experimental findings.