The phase-boundary potential between the moderately hydrophobic ionic liquid and a low ionic strength aqueous solution is demonstrated to be stable and constant with the standard deviation of 0.4 mV down to 20 mu mol kg(-1) HBr, LiBr, and KBr solutions, for three ionic liquids that consist of either N-methyl-N-octylpyrrolidinium, N-heptyl-N-methylpyrrolidinium, or N-hexyl-N-methylpyrrolidinium and a common anion species, bis(pentafluoroethanesulfonyl)amide. This stability is promising for accurate measurements of pH of low ionic strength samples and reliable estimates of single ion activities in general. The phase-boundary potential deviates from the value determined by the partition of the ionic liquid in further dilute aqueous solutions. The magnitude of the deviation ranges from 3 to 11 mV at 5 mu mol kg(-1) MBr (M is H(+), Li(+), or L(+)). The solubility of these ionic liquids in water is 0.2 mmol dm(-3) at most at 25 degrees C, which is another advantage of ionic liquid salt bridge in electroanalytical chemistry. (c) 2010 Elsevier B.V. All rights reserved.