An iron-antimony-molybdenum-containing oxide catalyst composition for oxidation reactions, comprising a crystalline iron antimonate having a crystallite size of 100 .ANG. or more, said catalyst being represented by the following empirical formula:Fe.sub.a Sb.sub.b Mo.sub.c L.sub.d K.sub.e M.sub.m N.sub.n Q.sub.q R.sub.r T.sub.t O.sub.xwherein L represents at least one element selected from the group consisting of Bi and Te; M represents at least one element selected from the group consisting of Cu, Mg, Zn, La, Ce, Al, Cr, Mn, Co, Ni and Sn; N represents at least one element selected from the group consisting of Be, Ca, Sr, Ba, Y, Pr, Nd, Th, U, Ti, Zr, Hf, Nb, Ta, Re, Ru, Os, Rh, Ir, Pd, Pt, Ag, Au, Cd, Al, Ga, In, Ge and Pb; Q represents at least one element selected from the group consisting of V and W; R represents at least one element selected from the group consisting of Li, Na, Rb, Cs, and Tl; T represents at least one element selected from the group consisting of B, P, As and Se; a, b, c, d, e, m, n, q, r, t and x each is an atomic ratio, wherein when a is 10, b is 5 to 60, c is 5 to 30, d is 0.01 to 10, e is 0.01 to 5, m is 0 to 30, n is 0 to 10, q is 0 to 5, r is 0 to 3, t is 0 to 5, and x is a number of oxygen atoms as determined corresponding to the oxides formed by combining the above mentioned components.