A process for the preparation of phosphonomethylglycine in nearly quantitative yields which eliminates the use of a noble metal co-catalyst and its concomitant problems. The oxygen-containing gas is replaced with the much more chemically active hydrogen peroxide, which in combination with activated carbon, oxidizes the PMIDA to PMG in very high yield. The amount of oxidant allowed to react with PMIDA is a function of the concentration of oxygen in the gaseous reaction products, and is controlled by monitoring that concentration. In another embodiment, the oxidizing agent is an oxygen-containing gas. In either case, the concentration of oxygen in the gaseous reaction product is monitored to determine the end point of the reaction.