The kinetic of conjugated reactions of S-oxidation and perhydrolysis of phenoxymethylpenicillin with potassium hydrogenperoxomonosulfate in alkaline medium has been studied by the light absorbance increase of a forming product at 269 nm. The procedure of the quantitative analysis of phenoxymethylpenicillin in powder and drug by the kinetic spectrophotometric method is elaborated using triple potassium Caro salt solution («ÃƒÂÃ‚žxon») as a reagent. RSD ≤ 1.9 %.