Catalyzed vapor-phase processes are taught for the oxidative dehydrogenation of a lower-alkyl-monosubstituted ethylbenzene to a lower-alkyl-monosubstituted styrene. Barium pyrophosphate and the pyrophosphate-containing calcination product of KFe.sub.3 H.sub.14 (PO.sub.4).sub.8.nH.sub.2 O, n running between about 1 to about 4, are shown to effectively catalyze these dehydrogenations at a low enough temperature such that very little cracking of the lower alkyl group occurs which gives superior conversions and selectivities to the corresponding styrenes and lengthened catalyst lifetime. An improved method of preparation of KFe.sub.3 H.sub.14 (PO.sub.4).nH.sub.2 O is described as well as the new material which is essentially KFe.sub.3 H.sub.6 (P.sub.2 O.sub.7).sub.4.