Betaines of formula R3N+-Q-COO− (I), wherein R is C1-4 alkyl and Q is C1-4 alkanediyl, optionally substituted with hydroxy, are prepared in one step by adding an ω-halocarboxylate of formula X-Q-COOR′ (II), wherein Q is as defined above, R′ is Cl1-4 alkyl and X is chlorine, bromine or iodine, to an aqueous solution containing a tertiary amine of formula R3N (III), Wherein R is as defined above and a base selected from alkali hydroxides and alkaline earth hydroxides. The process is particularly suited to the production of L-carnitine.