A process for the preparation of an optionally substituted trifluoromethyl-naphthalene which comprises contacting a compound of the formula ##STR1## in which R.sup.1 denotes hydrogen, alkyl, aralkyl, aryl, aryloxy, arylthio, polyhalogenoalkoxy, polyhalogenoalkylthio, halogen, nitro, halogenocarbonyl, halogenosulphonyl, alkylsulphonyl or arylsulphonyl, it being possible for the aromatic nuclei contained in the substituents R.sup.1 to be in turn substituted by halogen, alkyl, polyhalogenoalkoxy or polyhalogenoalkylthio,R.sup.2 represents hydrogen, halogen or alkyl andR.sup.3, R.sup.4, R.sup.5 and R.sup.6 denote hydrogen, orR.sup.3 and R.sup.4 or R.sup.4 and R.sup.5 in each case together denote a fused-on aromatic ring, and wherein, in the case where R.sup.3 and R.sup.4 together form a fused-on aromatic ring,R.sup.5 and R.sup.6 independently of one another can also denote, in addition to hydrogen, halogen, nitro, halogenocarbonyl or halogenosulphonyl, and wherein, in the case where R.sup.4 and R.sup.5 together form a fused-on aromatic ring,R.sup.3 and R.sup.6 independently of one another can also denote, in addition to hydrogen, halogen, nitro, halogenocarbonyl or halogenosulphonyl,with carbon tetrachloride and hydrogen fluoride at a temperature of 70.degree. to 120.degree. C. and under a pressure of 5 to 30 bars the reaction product then optionally being treated with catalyst activated hydrogen.