The present invention relates to substituted pyrimidine derivatives as well as N-oxides and agriculturally acceptable salts thereof, and their use to control undesired plant growth, in particular in crops of useful plants. The invention extends to herbicidal compositions comprising such compounds, N-oxides and/or salts as well as mixtures of the same with one or more further active ingredient (such as, for example, an herbicide, fungicide, insecticide and/or plant growth regulator) and/or a safener.