Oxidative esterification of an aldehyde and an alcohol on a heterogeneous catalyst is a green and mild alternative to classic routes to esters. The reaction of methacrolein with methanol and oxygen to methyl methacrylate is considered. A fixed-bed catalyst which has a well-defined core/shell structure and pore size distribution was applied instead of a typically used powder catalyst. It was charged in a fixed-bed reactor with a recycle for liquids, while the oxygen-containing gas was fed continuously. The mass transport of the multiphase reaction system is investigated and the results show the different influences like mass transfer through phase boundaries and diffusion limitations of the oxygen mass transport. Different ways are proposed to increase the oxygen mass transfer and decrease diffusion limitations.