A modified FETAX (Frog Embryo Teratogenesis Assay: Xenopus) protocol was used to assess the joint action of ten aliphatic carboxylic acids on Xenopus embryo development. Stock solutions of each acid alone, made up at twice the EC50 of the individual acids, were prepared for testing alone and in a mixture with an equal volume of each acid stock solution. For each treatment, five concentrations and a control dish, each with 25 embryos, were tested for 96 h, with solution renewal every 24 h. The embryos were then fixed and evaluated for gross malformations. For each dish, the number and types of malformations were recorded. An EC50 was calculated for each acid alone and this value was defined as 1.0 toxic unit (TU) for malformation induced by the acid. An EC50 was also calculated for the mixture. The concentration of each acid at the mixture EC50 and the TU values corresponding to these concentrations were then determined. A TU value of 0.990 (0.923–1.060) was obtained for the mixture by adding the TU values for each acid in the mixture. This represents a concentration additive rate of malformation. Microcephaly, TU = 1.09 (1.01–1.18), was the primary malformation, but did not completely account for the response. The concentration additive rate of malformation indicates that all ten acids are likely to induce malformation in Xenopus embryos in a similar manner. Quantitative structure-activity relationship (QSAR) analysis revealed developmental malformation induced by the acids was highly correlated (r2 = 0.979) with hydrophobicity and molar refractivity (r2 = 0.949). The approach has potential application in determining compounds that induce developmental malformations in a similar manner, when metabolism and pharmacokinetic factors are considered.