Decarboxylation of substituted cinnamic acids having a hydroxyl group at the para position gave predominantly the corresponding styrene derivatives in the presence of base with microwave heating. The reaction was conducted either under solvent-free conditions or using a solvent. When a primary amine was used as a base, the yield of the styrene or amide depended on the substituent of the cinnamic acid. Microwave heating for this reaction suppressed the side reactions compared with conventional heating.