Methylcellulose (MC) is one of the widely used bio-based polymers derived froni cellulose. MC possesses an amphiphilic property, since it contains both the hydrophilic hydroxyl group and the hydrophobic methoxy group in its molecular structure. Due to the amphiphilicity, aqueous MC solution shows temperature-sensitive property i.e. the lower critical solution temperature (LCST) type of phase behavior. On the other hand, a special organic salt, sodium tetraphenylborate (NaBPh4) also possesses an amphiphilic property, since it consists of hydrophilic cation Na+ and hydrophobic anion BPh4-. Motivated by an interest in the combined effect of the dual amphiphilicity, here we studied the addition effects of NaBPh4 on the cloud point of aqueous MC. When the additive amount of NaBPh4 to aqueous MC was small, the cloud point was depressed with the amount of NaBPh4, namely the salting-out effect. However, when the additive amount of NaBPh4 exceeded a certain value, the cloud point was elevated drastically with the amount of NaBPh4, namely the salting-in effect. We proposed a hypothesis that the switching from the salting-out to the salting-in is caused by concentration-specific binding of NaBPh4 to MC; NaBPh4 mostly dissociates into Na+ and BPh4- acting as the salting-out salt when the concentration of NaBPh4 is small, whereas the hydrophobic anions BPh4- are bound to the hydrophobic region of MC acting like a surfactant when the concentration of NaBPh4 exceeds a critical value. The presence of such a concentration-specific binding of anions was confirmed by the electric conductivity measurements. (C) 2016 Elsevier Ltd. All rights reserved.