A process is provided for obtaining sulfophenyl quarternary ammonium or phosphonium substituted carbonic acid esters. The important step in this process involves reacting a phenyl quaternary ammonium or phosphonium substituted carbonic ester with oleum that contains from 0.5 to 33% sulfur trioxide. Amounts in excess of 33% sulfur trioxide lead to increased reaction times and, eventually, unacceptable product conversons.