This invention relates to a process for preparing thioethers by the addition of an SH group from a thiol-containing compound, e.g., an alcohol, ester or acid, across at least one double bond of an ethylenically unsaturated compound in the presence of an initiator comprising a pinacol of the general formula: ##STR1## wherein R is phenyl or substituted phenyl, said substituents being members of the group consisting of halogen, alkyloxy and alkyl, said alkyloxy and alkyl containing 1 to 6 carbon atoms.