Silver mellitate decomposed, without melting, on heating in a vacuum at 195–250° giving a carbon dioxide yield corresponding to ∼75% of the constituent oxygen of the salt. Metallic silver was the only crystalline product identified. Kinetic measurements were made on two preparations, Salts A and B, which consisted of irregular crystallites and well-formed platelets respectively. Fractional decomposition (α)–time plots for Salt A showed an initial deceleratory reaction, followed by a sigmoid shaped curve. The most important kinetic feature of the decomposition of Salt B was pronounced zero-order reaction when 0·14 < α < 0·83. The activation energy for the main reaction in both salts was 40·0 ± 2·0 kcal. mole–1. These observations are interpreted by a model in which the solid phase reaction involved rapid initial formation of large numbers of small nuclei on crystallite surfaces with subsequent progression of the interface into the undecomposed reactant.