A method for preparing methyl methacrylate from methacrolein and methanol. The process comprises contacting in a reactor a mixture comprising methacrolein, methanol and oxygen with a heterogeneous catalyst comprising a support and a noble metal, wherein said catalyst has an average diameter of at least 200 microns, liquid and gaseous reactants flow downward in the reactor and wherein the continuous phase in the reactor is a gas which has no more than 7.5 mol % oxygen at reactor inlets.