Abstract: A process for the selective preparation of 2,4-dichloro-6-aryl- or 2-chloro-4,6-diaryl-1,3,5-triazines is described, which comprises reacting a compound of formula II ##STR1## wherein X is a bromine or chlorine atom, and R.sup.1 and R.sup.2 are independently hydrogen or alkyl, in tetrahydrofuran with magnesium metal to form the corresponding Grignard reagent, and reacting the resultant solution with cyanuric chloride of formula III ##STR2## such that, if Z is --Cl, at least 1.05 mol and, if Z is a radical of formula Ib, at least 2.1 mol, of Grignard reagent is used per 1 mol of cyanuric chloride. The compounds of formula I can be used with advantage for the preparation of 2-(o-hydroxyphenyl)-4,6-diaryl-s-triazines, which are known light stabilizers for organic material.