A new, solid acid-catalyzed microwave-assisted environmentally benign synthesis of isobenzofuran-1(3H)-ones is described. Montmorillonite K-10 appeared to be an excellent catalyst for the condensation and successive lactonization reactions. Reaction of phthalaldehydic acid (2-carboxybenzaldehyde) with methylaryl and cyclic ketones was initiated by microwave irradiation and occurred in one step. The reactions were complete in 10–30 minutes providing excellent yields (90–98%).