Polyhalogenated cinnamic acids and cinnamic acid derivatives are prepared by reacting diazonium salts accessible from polyhalogenated anilines with acrylic acid or acrylic acid derivatives in the presence of a homogeneous, palladium-containing catalyst at about −5 to about +100° C. Some of the cinnamic acids and cinnamic acid derivatives obtainable in this way are new. Cinnamic acids and cinnamic acid derivatives which can be prepared according to the invention can be used for the preparation of indanones which are precursors for agro- and pharmaceutical chemicals and for substances having liquid-crystalline properties.