Abstract: The invention relates to the batch or continuous para-chlorination of para-unsubstituted phenols, especially ortho-cresol, using sulphuryl chloride in the presence of one or more rate and selectivity catalysts, and to the avoidance of the reaction mixture going solid. It concerns specifically the carrying out of the chlorination process in stages, each stage involving only part of the sulphuryl chloride needed for the chlorination and being carried to completion before there is added to the mixture the sulphuryl chloride for the next stage. In addition, the invention concerns specifically the selection and control of reaction temperature, throughout the reaction, in order to minimize by-product formation and the likelihood of freeze-up.