6-(Carbo-t-butoxy)-; 6-(carbo-t-butoxymethyl)-; and 6-(carbo-t-butoxyethyl)-3-mercapto-4-acetamino-1,2,4-triazin-5-ones are prepared by conversion of the corresponding carboxylic acids via their acetic anhydride reaction products to their t-butyl esters by reaction with t-butyl alcohol. They are useful as herbicides.