We use a monomer-single-multimer model to judge whether there is significant self-association of an organic solute distributed between 1-octanol and water. Self-association leads to octanol-water partition coefficients, Kow, that depend upon the concentration of solute and this affects their application, significantly so for lipophilic compounds. Our measurements, done over as wide a range of concentration as possible, suggest that: (1) For toluene, there is dimerization in the water and tetramerization in 1-octanol. (2) For p-xylene, there is significant self-association of unknown degree in 1-octanol. (3) Biphenyl exhibits no self-association in either phase. The model confirms the conclusion that there is self-association of nicotimamide only in the aqueous phase, a conclusion reached in original measurements and interpretation (Charman, W. N.; Lai, C. S. C.; Finnin, B. C.; Reed, B. L. Pharm. Res. 1991, 8, 1144-1150). Our analysis of published measurements on the four isomers of hexachlorocyclohexanes (Paschke, A.; Shüürmann, G. Chem. Eng. Technol. 2000, 23, 666-670) leads to the conclusion that there is significant self-association of unknown degree in the aqueous phase. There is a discernible region of concentration-independent behavior as infinite dilution is approached in the aqueous phase, except notably for the hexachlorocyclohexanes. We suggest this is due to self-association incorporating the solvent to form multimer-solvent complexes. The data suggest that self-association, when it occurs, has a greater significance in the more lipophilic cases and this may partly explain why the variability in measurements of octanol-water partition coefficients between laboratories tends to be larger and significant for more lipophilic solutes.