To obtain 2-quinolinylthioacetic acid which is an intermediate product for producing 2-tribromomethylsulfonylquinoline useful as an effective stabilizer for photothermographischer materials and having excellent skin affinity, in a high yield and in a high purity by one step comprising the direct reaction of 2-chloroquinoline with mercaptoacetic acid.

This method for producing 2-quinolinylthioacetic acid formula II or its hydrochloride salt comprises reacting 2-chloroquinoline of formula I with mercaptoacetic acid in an aqueous solution state and, if necessary, converting the obtained salt into the free acid or the free acid and a salt. The method for producing 2-quinolinylthioacetic acid formula II or its hydrochloride salt comprises, for example, melting 2-chloroquinoline at 60 to 70°C, adding NaOH and tetrabutylammonium iodide, adding mercaptoacetic acid as its NaOH aqueous solution, reacting the compounds under refluxing for a long time, cooling the reaction solution, washing the solution with tertiary butyl methyl ether, and then adding concentrated hydrochloric acid to the aqueous phase to precipitate the hydrochloride. The precipitation process is preferably carried out in an acidic region, namely, at a pH value of <1, and the precipitates are preferably washed with distilled water or the like and then dried.