Abstract: Substantially dust-free granules of tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) are prepared by reacting methylamine (II) with carbon disulfide (III) and formaldehyde (IV) or by reacting the methylammonium salt of N-methyldithiocarbamic acid (V) with formaldehyde (IV) by a process in which the reaction is carried out in the presence of at least one diaminoalkylene of the formula VIR.sup.1 -NH-A-NH-R.sup.2 VIwhere R.sup.1 and R.sup.2 independently of one another are each hydrogen or alkyl and A is an unsubstituted or substituted 1,2-ethylene, 1,3-propylene or 1,4-butylene bridge.