The hydrolysis of bis (perfluoroalkanoyl) peroxides such as bis (perfluoroacetyl) peroxide [(CF3CO2)2] and bis (perfluorooctanoyl) peroxide [(C7F15CO2)2] was conducted. Kinetic studies showed the decomposition of bis (perfluoroacetyl) peroxide to be remarkably accelerated by the addition of water, whereas that of bis (perfluorooctanoyl) peroxide to be only slightly accelerated. There thus appears to be strong repulsion for water owing to the high surface-active property of long-chain perfluoroalkyl groups (perfluoroheptyl group).