In a preceding study, 4,4-dithiodipyridine (DTDP) was shown to be superior to 5,5-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) for spectrophotometric measurement of thiol groups in aqueous solution. (i) Sensitivity is higher because a larger absorbance increase is seen at a given thiol concentration. (ii) The intrinsic reactivity of thiolate anions for DTDP is much higher than for Ellman's reagent; thus, the reaction can be carried out at pH > 4.5 instead of at pH 8.0. In the present study, these advantages of DTDP were exploited for spectrophotometric measurement of thiols in organic solvent. DTDP was found to quantitatively react with nonpolar thiols when triethylamine was used as catalyst, intense light absorption (between 344 and 360 nm) was seen when the reaction was terminated with acetic acid, and the spectrophotometric responses were independent of the nonthiol portions of the mercaptans. The determination limit (10؋ the standard deviation of the reagent blank) was 3 M, and the upper limit was ϳ40 M on a typical spectrophotometer. The thiol contents of the mercaptans were independently verified by a modification of standard iodometry in which toluene/butanol or chloroform/butanol was included to dissolve nonpolar mercaptans.