Abstract The reaction between mercuric oxycyanide and hydrochloric acid, in the presence of certain salts, has been examined. The reaction between potassium thiocyanate and mercuric oxide in aqueous solution appears to go to completion. The reaction between mercuric cyanide and potassium iodide (or sodium thiosulphate) is complete in the presence of formaldehyde. These reactions form the basis of a method for the determination of mercuric oxycyanide which is considered more satisfactory and accurate than that in the British Pharmacopoeia, 1953.