Discontinuous precipitation involves formation of a two phase aggregate from a supersaturated solid solution behind a migrating boundary. It is established that the solute transport occurs primarily through the migrating boundary, called the reaction front. This report presents a systematic study of discontinuous precipitation in a Zn-Ag alloy and measurement of grain boundary chemical diffusivity of Ag in Zn-Ag using a suitable analytical model for the first time. The necessary kinetic parameters were determined by optical and scanning electron microscopes. The activation energy for boundary chemical diffusion of Ag in Zn-Ag has been estimated to be 65.8 kJ/mol.