PURPOSE: To obtain the titled adduct which is a flame-retardant and an epoxy resin hardener in high purity and yield, by reacting melamine with cyanuric acid at least one of which is in the solid state in an aqueous solvent in a specific pH range of the reaction system and at a high reaction rate.

CONSTITUTION: Melamine is reacted with cyanuric acid or isocyanuric acid in an aqueous solvent, e.g. water, a 1W4C alcohol or a mixture thereof, at a pH of the reaction system >7.0, preferably 8.0W10.5, and preferably under ordinary pressure and at 40W80°C, in a state wherein one or both of the reactants are dispersed in a solid state to give the aimed melamine-cyanuric acid adduct.