Polyaniline-sulfate salts were prepared using four different oxidizing agents such as benzoyl peroxide, ammonium persulfate, sodium persulfate and potassium dichromate. Polyaniline-sulfate salts were characterized by spectral, physical and electrical methods. Polyaniline base was redoped to different polyaniline salts using various acids. Polyaniline salts are used as polymer supported acid catalysts for the preparation of 7-hydroxy-4-methyl coumarin. The efficiency of the catalyst may be depend upon the amount of acid present on the polymer chain and also the strength of the acid used. The catalytic use of polyaniline salts is feasible because of their easy preparation, stability, easy handling, less toxic, recovery, reusability and eco-friendly. (C) 2003 Elsevier B.V. All rights reserved.