The present invention relates to substituted pyrimidine derivatives, as well as N- oxides thereof 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. The invention further relates to intermediates useful in the preparation of such compounds, and to processes for their preparation.