The Fries rearrangement of phenyl acetate for the paracetamol process is usually performed in hydrofluoric acid (HF). We have optimized this reaction with methane sulfonic acid (MSA), a strong, stable and biodegradable acid, to give para-hydroxyacetophenone with high conversion and selectivity (up to 92% of para-isomer and 8% of ortho-isomer at 100% conversion). (C) 2002 Elsevier Science B.V. All rights reserved.