Carboxylic acids have been conspicuously absent from the quantitative structure activity relationship (QSAR) literature. This study investigated the aquatic toxicity (log(IGC−150)) of selected mono- and di-carboxylic acids and their sodium, or disodium salts, tested in theTetrahymenapopulation growth assay. The relationship between log(IGC−150) and hydrophobicity as described by the 1-octanol/water partition coefficient (logKow) revealed a distinct sub-class. The relationship [log(IGC−150) = 0.27(logKow) – 0.68;n= 16,r2= 0.943.s= 0.07,F= 233, Pr>F= 0.0001] was derived for mono-carboxylic acids. The QSAR [log(IGC−150) = 0.19(logKow) – 0.66;n= 9,r2= 0.951,s= 0.08,f= 135, Pr <F= 0.0001] was generated for the di-carboxylic acids. Regression analysis of data for the monosodium carboxylic acid salts yielded the model, log(IGC−150) = 0.60 (logKow) + 0.58;n= 4,r2= 0.932,s= 0.19,F= 41.2, Pr <F= 0.008. Values for the ionization constant (pKa) and the energy of the lowest unoccupied molecular orbital (ELUMO) do not vary within the sub-class for saturated acids. Moreover,pKuandELUMOdid not describe differences in toxicity between sub-classes of saturated aliphatic carboxylic acids and salts. However, these descriptors did vary for unsaturated acids. Inclusion of unsaturated acids afforded the derivation of a global response-surface for all aliphatic carboxylic acids, log(IGC−150) = 0.25(logKow) – 0.13(ELUMO) – 0.54;n= 34,r2= 0.850,s= 0.138,F= 87.9, Pr <F= 0.0001. Outliers to the response-surface included small molecules that provided 2-positions in which the molecule could potentially undergo electrophilic attacked and other more large, hydrophobic molecules.