A one-pot, solid-acid-catalyzed, microwave-assisted synthesis of phthalazinones is described. The commercially available montmorillonite K-10 effectively catalyzed the condensation and substitution reactions. The approach was based on the direct cyclization of phthalaldehydic acid and opianic acid with substituted hydrazines. The reactions provided excellent yields and high selectivities in very short time (5-35 minutes).