Abstract: A process for the preparation of an unsaturated carboxylic acid which comprises reacting an unsaturated aldehyde with molecular oxygen in the vapor phase in the presence of an oxidation catalyst composition having the following empirical formulaMo.sub.a P.sub.b L.sub.c T.sub.d O.sub.ewherein L is at least one element selected from the group consisting of Rb, Cs and K; T is at least one element selected from the group consisting of Ba, Cd, W, Bi, Nb, B, Pb and Pd; and a, b, c, d, and e each represent the number of atoms of each element; the atomic ratio of a:b:c:d is 12:0.1-8:0.1-8:0.1-8 and e is the number of oxygen atoms determined by the valence requirement of the other elements present.