3 - PHENYL - 7 - (4''-CHLOROPYRAZOLYL-(1''))-COUMARINS WHICH BEAR FURTHER SUBSTITUENTS ESPECIALLY AT THE 3''AND 5''-POSITION OF THE PYRAZOLYL NUCLEUS, AND DESCRIBED AS OPTICAL BRIGHTENERS AFFORDING SUPERIOR DEGREE OF WHITENESS AND FLUORESCENCE NUMBER AS WELL AS BETTER FASTNESS TO CHLORITE BLEACHING BATHS ON SUCH SUBSTATES AS POLYESTER AND OTHER POLYMERIC ORGANIC MATERIALS. A PREFERRED COMPOUND IS 3-PHENYL-7-(3''-METHYL-4''-CHLOROPYRAZOLYL-(1'')COUMARIN. A METHOD OF BRIGHTENING POLYMERIC MATERIALS, E.G. FIBERSS ARE FILMS WITH THE WORD COMPOUND AS WELL AS POLYMERIC MATERIALS BRIGHTENED THEREWITH ARE ALSO DISCLOSED.