The trimethylsulphonium salt of N-phosphonomethylglycine is prepared by reacting N-phosphonomethylglycine, preferably in the form of a solid, with an aqueous solution of trimethylsulphonium carbonate or trimethylsulphonium bicarbonate or a mixture thereof. The trimethylsulphonium carbonate or trimethylsulphonium bicarbonate may be prepared by bubbling carbon dioxide through an aqueous solution of trimethylsulphonium hydroxide. It is possible to manufacture trimethylsulphonium carbonate or bicarbonate at one site and then transport them to a different site for local manufacture of N-phosphonomethylglycine in a relatively unsophisticated plant which produces minimal effluent.