The invention relates to the preparation of alkylthiophenols by reaction of a dialkyl disulphide with a phenol. In the process according to the invention, the reaction is carried out in the presence of aluminum chloride or of ferric chloride in a solvent of the alkylbenzene type or, soley in the case of methylthiolation, in an excess of dimethyl disulphide. This process makes it possible, in particular to obtain, with a selectivity and in a yield which are excellent, 2-alkylthiophenols which may then be converted into 4-acyl-2-alkylthiophenols by means of a reaction at a temperature ranging from 40.degree. to 100.degree. C. with a complex BF.sub.3 :2RCOOH where R denotes an alkyl or propenyl radical, in a proportion of 10 to 15 moles of complex per mole of 2-alkylthiophenol.

Compounds of the formula I, II or III ##STR1## in which Ar is a sulfur-containing aromatic radical, R.sup.1 and R.sup.2 are a monovalent hydrocarbon radical which is substituted or unsubstituted or R.sup.1 and R.sup.2 together form alkylene, oxaalkylene or azaalkylene, R.sup.3 is a direct bond or a divalent hydrocarbon radical, X is a monovalent amino group and X' is a divalent amino or diamino group, are excellent photoinitiators for the photocuring of colored, in particular pigmented, compositions containing an olefinically unsaturated, photopolymerizable binder.