A water soluble effervescent disinfecting tablet comprises a chlorinated isocyanurate such as anhydrous sodium dichloroisocyanurate. The tablet also comprises an alkali metal bicarbonate such as sodium bicarbonate, an aliphatic carboxyl acid such as adipidic acid, and a dessicant base such as sodium carbonate. The tablet contains a masking agent, especially isoamyl acetate and/or L-methone in an amount to mask taste and/or odour characteristics of the chlorinated isocyanurate.