Surface tensions (ST), distribution ratios, and interfacial tensions (IFT) for octanoic acid-octylamine mixtures, dissolved either in water or in isooctane-water systems, have been investigated. Distribution ratios were obtained by determining the concentration of octanoic acid and octylamine using FTIR spectroscopy and latent variable regression. The dependence of the measured parameters on the molar ratio between octanoic acid and octylamine, and on the aqueous phase pH, was also studied. The results showed that the simultaneous presence of protonated octylamine and dissociated octanoic acid had a significant effect on ST as compared to when either of the components was present alone. The effect was largest for solutions of equimolar composition. IFT was significantly lower in systems containing the octanoic acid:octylamine mixture as compared to systems containing only one of the components. The dependence of IFT on pH was only significant for the mixed systems. In these systems a minimum in IFT was found around pH 7, and the variation in IFT with pH corresponded roughly to changes in the distribution ratios with pH.