Abstract Diprotic acid dyes [H 2A: Bromophenol Blue (BPB), Bromochlorophenol Blue (BCPB), Bromocresol Purple (BCP), Bromocresol Green (BCG), Bromothymol Blue (BTB)] can be extracted as 1:1 ion-associates Q +HA − and 1:2 ion-associates (Q +) 2A 2− with quaternary ammonium cations (Q +) into chloroform. The extraction constants (log K ex) of the 1:1 and 1:2 ion-associates have been determined. Linear relationships between log K ex and the number of methylene groups in the quaternary ammonium ions were observed; from the slope of the line, the contribution of a methylene group to log K ex was found to be 0̃.43–0.65. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetra-alkylammonium cations, for both the 1:1 and 1:2 ion-associates. From the extraction constants obtained, the extractability of acidic dyes was in the order BTB > BCG > BPB > BCPB > BCP.