Abstract: For the oxidative dehydrogenation of saturated carboxylic acids to form .alpha.,.beta.-unsaturated carboxylic acids, saturated acids are contacted with a gas containing molecular oxygen and, where appropriate, an inert diluent gas, in the vapor phase, at a reaction temperature of 250.degree. to 600.degree. C., in the presence of a catalyst system comprising a non-supported catalyst of the formula FeP.sub.x Me.sub.y O.sub.z, wherein: Me is at least one of the following elements: Li, Na, K,, Rb, Cs, Mg, Ca, Sr and Ba; x has a value from 0.2 to 3.0; y has a value from 0.01 to 0.2; and z is the quantity of oxygen bonded to the other elements and corresponding to their oxidation state, in an entrained-bed reactor and, where appropriate, in the presence of steam in a molar ratio to the saturated acid not exceeding 0.5. The catalyst is beneficially associated with at least one zeolite, preferably in a concentration sufficient to improve the attrition resistance of the catalyst, e.g.