Abstract: A method of determination of sulfur dioxide in beer with iodometry by using formaldehyde as masking reagent is reported.The optimal conditions are as follows:pH is 3~4,the amount of formaldehyde is 2 to 4 times over the amount of sulfur dioxide,masking time is 4 min.,and tempeature is 25~33 ℃.The detection limit is 2×10 -5 g/kg.This method can be used to determine the sulfur dioxide in beer satisfactorily.