Abstract A flow injection system was proposed to evaluate the transient product of a colorimetric reaction between hydrolysable tannin and potassium iodate (KIO 3 ) solution. The system optimization was accomplished by using statistical methods based on experimental design. Flow rate of KIO 3 solution, sample volume, carrier flow rate, and reaction coil were the selected factors for evaluation. On screening step, complete factorial 2 4 was used and two levels for each selected factor were studied. For the optimization phase, a centered face composite design 2 2 + star was employed to evaluate sample volume and flow rate of KIO 3 solution, which were the factors identified in the screening phase as having more influence on the absorbance signal. After optimization, the proposed system was compared with batch determination. Some characteristics, such as analytical frequency, reagent consumption and chemical residues generation presented better results by the use of the proposed system if compared with batch method. The system presented good repeatability with standard deviation lower than 3%, for n = 10, linearity ( R 2 = 0.9974) for tannic acid standard, analytical frequency of 15 injections h − 1 and limit of quantification of 24 mg L − 1 of tannic acid. Good results were obtained when the proposed system was applied to hydrolysable tannin determination in Stryphnodendron barbatimao , Eucalyptus citriodora and Phyllanthus niruri , samples of plants commonly used in popular medicine.