A new kind of UV stabilizers, 1-(3′-(benzotriazol-2"-yl)-4′-hydroxy- benzoyl)-3-methyl-5-pyrazolones (1a-d), was synthesized with the aim to bind them chemically to certain polymers. The reaction of 1d with substituted benzaldehydes 4 in the molten state at 150℃ and in the solid state at room temperature produced the condensation products 1-( 3′-( 5″-chlorobenzotriazol-2"-yl)-4′-hydroxyl-5′-chlorobenzoyl)-3-me-thyl-4-arylmethylene-5-pyrazolones (2) and 4, 4′-arylmethy-lene- bis[ 1-(3′-( 5″-chloro-benzotriazol-2″-yl )-4′-hydroxy-5′-chloro-benzoyl)-3-methyl-5-pyrazolone] s (3), respectively,as the major product. On the other hand, the reaction of 1d with 4 at 50℃ in chloroform solution proceeded non-selectively to give a mixture of 2 and 3.