Abstract: Compounds of the formula ##STR1## are described in which R.sub.1 represents a perfluoroalkyl radical of from 1 to 2 carbon atoms, and at least one of R.sub.2 and R.sub.3 represents hydrogen, and the other, or both represent lower alkyl, and R.sub.4 represents a lower alkyl radical which may be unsubstituted or bear certain substitution, and which are useful as herbicides in a method for controlling weeds and wild grasses. Compositions containing such compounds are also described. Finally, novel fluoro-thio semicarbazides are described which are useful intermediates in the production of the compounds of Formula I.