The Fries rearrangement of aromatic esters is usually performed in Lewis acid (AlCl3), we have optimized this reaction with eco-friendly catalyst p-toluene sulphonic acid (PTSA) in situ. It was found to be as efficient new reagent for probing the mechanism of acylation reactions and Fries rearrangement of aromatic esters. PTSA is a strong, stable and biodegradable acid giving high conversion and selectivity (up to 90% of ortho-isomer and 10% of para-isomer at 100% conversion), Further, the conversion was confirmed by elemental and spectral Infra-red , Nuclear Magnetic Resonance, and Mass Spectroscopy (IR, 1H NMR Mass) analyses.