This invention is a process for the preparation of a pyrocarbonic acid diester of formula (I) ##STR1## wherein R.sub.1 and R.sub.1 ' are each, independently of the other, an optionally substituted branched or straight-chain C.sub.1 -C.sub.24 alkyl, C.sub.3 -C.sub.24 alkenyl, C.sub.3 -C.sub.24 alkynyl, C.sub.4 -C.sub.12 cycloalkyl, C.sub.4 -C.sub.12 cycloalkenyl or C.sub.7 -C.sub.24 aralkyl, by reacting at least one ester carbonate of formula (II) ##STR2## with 40-50 mol % of a sulfochloride of formula (IV) ##STR3## in the presence of 0.8-5 mol % of a catalyst of formula (V) ##STR4## and with minor amounts of a heterocyclic aromatic amine in a nonpolar inert solvent, in which process the amount of heterocyclic aromatic amine is 1-3 mol % and the reaction is carried out in the temperature range from -10.degree. C. to +25.degree. C., all molar amounts being based on 100 mol % of ester carbonate of formula (II).