The invention concerns novel 4-phenyl-1,3-dioxan-5-ylalkenoic acid derivatives of the formula I having cis relative stereochemistry at positions 4 and 5 of the dioxane ring and wherein Ra and Rb are variously hydrogen, alkyl, halogenoalkyl, alkenyl, and optionally substituted aryl or arylalkyl, Rc is hydroxy, alkoxy or alkanesulphonamido, n is 1 or 2, A is ethylene or vinylene, Y is (2-5C)polymethylene optionally substituted by alkyl, and benzene ring B is optionally substituted phenyl, or, when Rc is hydroxy, a salt thereof. The acid derivatives antagonize one or more of the actions of thromboxane A.sub.2 (TXA.sub.2) and are expected to be of value in those disease conditions in which TXA.sub.2 is involved. The invention also provides pharmaceutical compositions containing an acid derivative of formula I, and processes for their chemical production.