[en] The distribution behavior of ion associates of the tetraiodobismate(III) anion, BiI₄^-, with eight quaternary ammonium cations between a chloroform and an aqueous phase was examined, and the extraction constants (log K_ex) were determined. The extractability with long-chain alkyltrimethylammonium cations (group I) is greater than that with symmetrical tetraalkylammonium cations (group II). The contributions of a methylene group in the cations of groups I and II to log K_ex were found to be about 0.54 and about 0.40 on the average, respectively. From the extraction constants obtained, a highly sensitive extraction-spectrophotometric method of bismuth was developed. (author)