AbstractThe vaporisation of sulphamic acid, NH2SO3H, at temperatures between 600 K and 740 K has been studied. The chemical constitution of the acid changes on melting in a manner which may be described by the equation: 12NH2SO3H→6NH4HSO4+ (NHSO2)6.SO3/H2SO4is released thereafter by the hydrolysis of the hexasulphimide (NHSO2)6. the requisite H2O being derived mainly from the decomposition of the NH4HSO4.SO3/H2SO4release is virtually complete by the time the acid has lost 20% of its original weight. During further vaporisation, SO2is the only vapour‐phase sulphur‐containing species to be released in significant quantities, the unevaporated residue comprising mainly of a 2:2:1 molar mixture of (NH4)2SO4, NH4HSO4and NH2(SO2NH)2SO2NH2.The use of sulphamic acid as a conditioner for the fly ash found in the flue gases of coal‐fired boiler plant is di