Kinetics and S-oxidation reaction of semisynthetic penicillins (amoxicillin trihydrate, ampicillin trihydrate, sodium oxacillin and disodium ticarcillin) by means of potassium hydrogenperoxomonosulphate in aqueous solutions at pH 3-6 was studied using iodometric titration. 1 mol of penicillin is per 1 mol of KHSO5, quantitative interaction proceeds in 1 minute (observation time). The unified procedure was developed and ability of quantitative determination of penicillin by iodometric method using potassium hydrogenperoxomonosulphate (KHSO5) as analytical reagent was shown. RSD ≤ 2. 35 % (δ = -0.1…+1.0%).