Sodium perborate in its effervescent form (ESP) has the ability to release both gaseous and active oxygen in aqueous solution. Gravimetric analysis and direct measurement of gaseous oxygen have been used to investigate the ideal conditions for the formation of ESP from sodium perborate monohydrate (PBS1). A dehydration temperature of 125 degrees C gave the maximum yield of effervescent oxygen, with the mass loss corresponding closely to 1 mol of water/mol of PBS1. In stability studies, freshly formed ESP was capable of generating 10.1 wt % effervescent oxygen. However, this value fell to only 1.35% of its initial value after exposure to ambient air (26 T and 36% relative humidity), confirming the unstable nature of this material. ESP stability was improved by screening its surface with a coating of fine boric acid powder. Further work on the coating of ESP is recommended in order to further improve its stability.