Cesium phosphomolybdate, 12MoO(3).Cs2HPO4, precipitated with Cs2CO3, dried 12 h/120degreesC, heated 6 h/180degreesC, 6 h/300degreesC in 120 L/h air, powdered, compressed to yield 0.25-0.5 mm particles, sp. surface 4.13 m(2)/g, preoxidized 1 h/523 K with O-2, was used to catalyze oxidative dehydrogenation in [x:14:(86-x) by moles] (x = 2, 3, 4) or (3:9:91) isobutyraldehyde (I)-O-2 -N-2 mixtures, 5 dm(3)/h (F), at 523 K over 0.02-1.5 g (w) catalyst in a fixed-bed integral reactor, time factor w/F = 14,400-1.08.10(6) g.s/m(3), to yield methacrolein and CO. At the start, the catalyzed reaction went together with homogeneous decomposition of I. In the T-programmed (TP) reaction (297-623 K; 10 K/min; F, 150 mL/min; I/O-2 mole ratio, 3/1, 1/1, 1/3), methacrolein appeared at 100degreesC and CO2 at 200degreesC. CO attained max. at 1000degreesC. TP desorption studies showed methacrolein to form on the catalyst surface by the Mars-van Krevelen redox mechanism. Keggin's structure (Cs surrounded by 12 molybdate octahedra) remained unaffected by any reaction conditions. The catalyst was active and selective up to 90% toward methacrolein at about 470 K. Adsorption of (a) / and (b) methacrolein on the fresh catalyst at 80, 100 and 120degreesC, best reproduced by Langmuir isotherms involving no dissociation of adsorbates, gave DeltaH(o) DeltaG(o) and DeltaS(o) at 298 K (a) -14.8. 33.8 kJ/mol and -163 J/(K mol), (b) -26.8, 28.8 kJ/mol and -185 J/(K mol), resp.