A modification of the iodine technique for peroxide determination is described, in which the reaction is carried out in deaerated methanol:acetic acid solution in a spectrophotometer cuvette. Measurements of the triiodide ions released are carried out either at 290 or 360 nm. In this solvent, the molar extinction coefficients are ε290 = 4.41 × 104 and ε360 = 2.80 × 104 liters mol-1 cm-1. Calibration with standard peroxide solutions showed that 1 mol of I3- is produced for every peroxide group added. In its present form, the method can be used to measure 1 nEq of lipid hydroperoxide.