Abstract: Improvements to a method for preparing N-phosphonomethylglycyne by oxidation of N-phosphonomethyliminodiacetic acid. N-phosphonomethylglycine may be obtained by oxidation of N-phosphonomethyliminodiacetic acid with O.sub.2 or with an oxygen-containing gas, in the presence of a noble metal catalyst on activated carbon. To avoid the catalyst losses, the improvements of the invention consist in a pressure scavaging with N.sub.2, after completion of the oxidation reaction. Owing to such treatment, the catalyst losses are reduced from 30% to 1%, making the processing economically viable on a large industrial scale. The recovered catalyst holds its catalytic activity and may be reused at least 20 consecutive times. N-phosphonomethylglycine is a herbicide of extended application.