A method for preparing 1-adamantyltrimethylammonium methylcarbonate or N,N-dimethyl-3,5-dimethylpiperidinium methylcarbonate is disclosed. The method comprises reacting dimethyl carbonate and 3,5-dimethylpiperidine or a 1-adamantylamine compound and in the presence of water in a sealed vessel at a temperature of from 80 to 200° C. The 1-adamantylamine compound is 1-adamantylamine, 1-adamantylmethylamine, or mixtures thereof.