The conversion of unsaturated acids to gamma-lactones is catalyzed by the solid sulfonic acid catalysts Amberlyst-15 and Nation SAC-13 in refluxing chlorobenzene. For 3- and 4-enoic acids, in which double bond migration is not required for gamma-lactone formation, yields are 95-100%. For 5-, 6-, 9-, and 10-enoic acids, the lactone yield decreases from 95% to 25% as the distance between the olefin and carboxylic acid groups increases. Amberlyst-15 is the preferred catalyst in these reactions because of its lower cost and reusability. (C) 2007 Elsevier B.V. All rights reserved.