Apparent partition coefficient of methyl orange between H₂O and n-octanol at 25°C was found to be dependent on the concentrations of potassium or sodium ion in aqueous phase above pH 7.0, indicating that methyl orange is transferred from aqueous to organic phase via an ion-pair complex for mation with these cations. Potassium and sodium ions had the same affinity for methyl orange, and the hydrophobicity of potassium-methyl orange complex was approximately the same as that of sodium-methyl orange complex. However, in strong acidic region the partition of methyl orange was found to be independent on the potassium ion concentration. It would be possible that in this region only a neutral molecular species of methyl orange is transferred from aqueous to organic solvent phase.