In the presence of N, N-dimethylformamide, the reaction between fluorescein and phenylsulfonyl chloride proceeds with two consecutive steps: first, fluorescein is esterified with phenylsulfonyl chloride, and chloride ion formed nucleophilically attacks aryl carbon and then substitutes for phenylsulfonyloxy, which is a good leaving group. Whole effect of the reaction is that the substitution of chlorine for phenolic hydroxy on fluorescein. The reaction mechanism is certificated by the key intermediate found in the work.

Key words: BENZENESULFONIC ACID P, MASS SPECTROGRAPHY, INTERMEDIATE, DIMETHYLFORMAMIDE, ACYL CHLORIDES P, LUCIFERIN, REACTION MECHANISM