compositions of the empirical formula : mo . sub . 12 p . sub . 0 . 1 - 3 m . sub . 0 . 1 - 3 cu . sub . 0 . 1 - 2 v . sub . 0 . 1 - 2 x . sub . 0 . 01 - 2 y a o b where m is at least one of k , rb and cs ; x is at least one of ba , la , ga , al , ag , cd , ti , tl , hg , pb and zn ; y is at least one of fe , co , ni , sr , mn , in , ta , ge , s and be when a & gt ; 0 ; a is a number of 0 to about 2 ; and b is a number that satisfies the valence requirements of the other elements present , are excellent catalysts for the oxidation of methacrolein to methacrylic acid .