Abstract: This invention relates to a novel catalyst for preparing methacrylic acid by gas phase oxidation of methacrolein obtained by gas phase catalytic oxidation of isobutylene or t-butanol. The catalyst is represented by the formula:P.sub.a Mo.sub.11 V.sub.b Du.sub.c X.sub.d Q.sub.e Z.sub.f O.sub.gwhereinX is at least one of potassium, rubium, cessium, and thallium;Z is at least one of lead, antimony, chromium, iron, bismuth, cerium, and zinc;Q is at least one of organic quaternary ammonium cations consisting of ##STR1## in which R, R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently selected from the group consisting of C.sub.1 -C.sub.5 alkyl or C.sub.1 -C.sub.5 substituted alkyl having functional groups;a is a number from 0.8 to 1.6;b is a number from 0.6 to 2;c is a number from 0.1 to 0.8;d is a number from 0.7 to 2.2;e is a number from 0.01 to 0.1;f is a number from 0 to 0.5; andg is a number of oxygens required to satisfy the valence requirements of the other elements present.