In the methylation of 4-amino-6-t.-butyl-3-mercapto-1,2,4-triazin-5-one to produce 4-amino-6-t.-butyl-3-methylmercapto-1,2,4-triazine-5-one using methyl bromide, the reaction is run in the absence of air whereby product coloration is avoided. Desirably considerable agitation is employed along with recycle and especially with an emulsifying agent which markedly speeds up the reaction. Measures such as operating under a slight vacuum and recycle of unused methyl bromide and mother liquor are employed to improve the yield and to avoid the problems of discharging the bromides to the environment.