For the preparation of 3-alkoxyphthalides from phthalaldehydes and alcohols, a cyclization reaction in the presence of a rhodium(III) catalyst and copper(II) salt is reported. Cyclization of phthalaldehydes also occurs with 1,3-dicarbonyl compounds under similar conditions to produce 3-substituted phthalides in good yields. An acylrhodium(III) species might be a key intermediate in these cyclization