A process for preparing S-containing pyrimidinium compound of formula X wherein C* is an asymmetric carbon atom of S or R-configuration; R 1 is C 1 -C 4 -alkyl, C 3 -C 6 -cycloalkyl, C 2 -C 4 -alkenyl, or -CH 2 -phenyl, which groups are unsubstituted or substituted with halogen or C 1 -C 4 -alkyl; R 2 is a 5-or 6-membered saturated, partially unsaturated or aromatic carbo- or heterocyclic ring, wherein the ring is unsubstituted or substituted with R 2a; Het is selected from D-1, D-2, and D-3: wherein R a is each independently R a is halogen, C 1 -C 4 -haloalkyl, C 1 C 4 -alkoxy, C 1 -C 4 -alkylthio, or phenyl; n is 0, 1 or 2; and # denotes the bond in formula X; R 2a is halogen, C 1 -C 6 -haloalkyl, C 1 -C 6 -haloalkoxy, OR c, C(=O)OR c, C(=O)NR b R c, phenyl, or pyridyl, which is unsubstituted or substituted with halogen, C 1 -C 6 -haloalkyl or C 1 -C 6 -haloalkoxy; R b is hydrogen, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy; R c is hydrogen, C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C 1 -C 6 cycloalkyl; wherein two geminally bound groups R c R b together with the atom to which they are bound, may form a 3- to 7- membered saturated, partially unsaturated or aromatic heterocyclic ring; comprising at least the step of, reacting the compound of formula VII, wherein C* is an asymmetric carbon atom of S or R-configuration; Het is as defined in compound of formula X; with R 1 NCS, wherein R 1 is C 1 -C 4 -alkyl, C 3 -C 6 -cycloalkyl, C 2 -C 4 -alkenyl or -CH 2 -phenyl, which groups unsubstituted or substituted by halogen or C 1 -C 4 -alkyl; in the presence of a base, to obtain a compound of formula VIII, wherein C* and Het are as defined in the compound of formula VII; R 1 is as defined herein. C* is an asymmetric carbon atom of S or R-configuration; R 1 is C 1 -C 4 -alkyl, C 3 -C 6 -cycloalkyl, C 2 -C 4 -alkenyl, or -CH 2 -phenyl, which groups are unsubstituted or substituted with halogen or C 1 -C 4 -alkyl; R 2 is a 5-or 6-membered saturated, partially unsaturated or aromatic carbo- or heterocyclic ring, wherein the ring is unsubstituted or substituted with R 2a; Het is selected from D-1, D-2, and D-3: wherein R a is each independently R a is halogen, C 1 -C 4 -haloalkyl, C 1 C 4 -alkoxy, C 1 -C 4 -alkylthio, or phenyl; n is 0, 1 or 2; and # denotes the bond in formula X; R a is each independently R a is halogen, C 1 -C 4 -haloalkyl, C 1 C 4 -alkoxy, C 1 -C 4 -alkylthio, or phenyl; n is 0, 1 or 2; and # denotes the bond in formula X; R 2a is halogen, C 1 -C 6 -haloalkyl, C 1 -C 6 -haloalkoxy, OR c, C(=O)OR c, C(=O)NR b R c, phenyl, or pyridyl, which is unsubstituted or substituted with halogen, C 1 -C 6 -haloalkyl or C 1 -C 6 -haloalkoxy; R b is hydrogen, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy; R c is hydrogen, C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C 1 -C 6 cycloalkyl; wherein two geminally bound groups R c R b together with the atom to which they are bound, may form a 3- to 7- membered saturated, partially unsaturated or aromatic heterocyclic ring; C* is an asymmetric carbon atom of S or R-configuration; Het is as defined in compound of formula X; C* and Het are as defined in the compound of formula VII; R 1 is as defined herein.