A preparative method was developed for the synthesis of dithiazinyl-substituted carboxylic acids by the cyclothiomethylation of amino acids and their derivatives with formaldehyde and hydrogen sulfide. The resulting substituted dithiazines are promising as biocides and selective complexing agents. The effect of the structure of the starting amino acids, reaction conditions, and reactant ratio on the yield and composition of cyclothiomethylation products was studied.