The application of C 7 aliphatic carboxylic acids (heptanoic, 2-methylhexanoic, 5-methylhexanoic and 2,2-dimethyl- n-valeric acids) as eluents in ion-exclusion chromatography with conductimetric detection for C 1–C 6 aliphatic carboxylic acids (formic, acetic, propionic, isobutyric, butyric, isovaleric, valeric, isocaproic and caproic acids) was carried out using a highly sulfonated styrene–divinylbenzene co-polymer resin (TSKgel SCX) in the H + form as a stationary phase. When using 0.05 m M sulfuric acid at pH 4.0 as the eluent, peak shapes of hydrophobic carboxylic acids (isovaleric, valeric, isocaproic and caproic acids) were tailed strongly. In contrast, when using 1 m M these C 7 carboxylic acids at pH ca. 4 as the eluents, although system peaks (vacant peaks) corresponding to these C 7 carboxylic acids appeared, peak shapes of these hydrophobic acids were improved drastically. Excellent simultaneous separation and relatively high sensitive conductimetric detection for these C 1–C 6 aliphatic carboxylic acids were achieved in 25 min on the TSKgel SCX column (150×6 mm I.D.) using 1 m M 5-methylhexanoic acid at pH 4.0 as the eluent.