A series of Cs(NH4)(x)H3-xPMo11VO40 with different x values were synthesized to catalyze the selective oxidation of methacrolein to methacrylic acid. The effects of ammonium on both structure and catalytic activity Were explored in this study. Compared with CsH3PMo11VO40, the surface area, amount of acid sites, and active species (V4+/VO2+) were found to be strongly dependent on the content of ammonium. The optimum structure of Cs(NH4)(1.5)H1.5PMo11VO40 shows comparable large surface area of 50.33 m(2)/g, large amount of acid sites, and active species (V4+/VO2+), consequently offering a higher methacrolein conversion (83%) and methacrylic acid selectivity (93%). Furthermore, the steady activity of Cs(NH4)(1.5)H1.5PMo11VO40 was not affected much by the decomposition of ammonium, and the catalyst exhibited good stability.