A needle-type micro liquid-junction reference electrode for general use was developed based on the novel use of a thin-film Ag/AgI electrode. The internal iodide ion concentration was maintained at a constant value determined by the solubility constant of AgI. Iodide ions in an external solution that might penetrate into the interior were eliminated. They were electrochemically converted into iodine molecules and trapped in a starch matrix that filled in the liquid junction. The sensitivities of the bare Ag/AgI electrode to interferents were 4.2, 2.8, and -6.1 mV/dec for L-ascorbic acid (50-200 mu M), uric acid (50-200 mu M), and NaCl (50-200 mM), respectively. The internal filling solution was rapidly exchanged, within several minutes, for step changes of the NaCl concentration which expanded 1 order of magnitude. When the liquid junction was active, changes that occurred in the electrode potential when the external iodide ion concentration changed by 1 order of magnitude in the nanomolar range were suppressed within several millivolts. When the liquid-junction reference electrode was used with a pH-indicator electrode in solutions containing different concentrations of various interferents, the achieved precision was +/-0.01 pH unit. The pH measurement could also be conducted in human serum solutions of only several microliters. (c) 2006 The Electrochemical Society.