The potentiometric responses of copper and nickel hexacyanoferrate membrane electrodes were examined for alkali, alkaline earth, heavy metal and ammonium ions. When K2Cu3[Fe(II)(CN)(6)](2) membranes were electrodeposited on a Cu plate in aqueous K-4[Fe(II)(CN)(6)] solution at applied potentials from +0.20 to +0.40 V vs SCE, the membranes exhibited a near-Nernstian response (55 mV/decade) to K+ ions above 1 x 10(-4) M. When this K2Cu3[Fe(II)(CN)(6)](2) membrane was immersed and conditioned in 0:1 M (CH3)(4)N . NO3 (tetramethylammonium nitrate, TMA . NO3) solution prior to potentiometric measurements, the preferential dissolutions of K+ ions into the adjacent solution was observed accompanying negative membrane potential shifts (more than 200 mV). The potentiometric response of copper hexacyanoferrate membranes electrodeposited above 0.40 V vs SCE became however weaker with decreasing K+ content in CuHCF membranes and both the amount of preferential K+ dissolution and the extent of the membrane potential shift became smaller during the membrane conditioning. From these results, it was concluded that negatively charged K+ vacancies at the membrane surface contacting the adjacent electrolyte solution (0.1 M (CH3)(4)N . NO3) were formed by the preferential K+ dissolution, into which the analyte K+ ions was back-titrated during the potentiometric response process. The potentiometric selectivities of the K2Cu3[Fe(II)(CN)(6)](2) and KNi[Fe(III)(CN)(6)] membrane electrodes for alkali metal ions were Cs+ > Rb+ > K+ > Na+ > Li+ and no significant responses were observed for any alkaline earth metal ions in the concentration range below 1.0 x 10(-2) M. These selectivities may reflect their dehydration energies needed for their accommodation from the solution side into the negatively charged vacancies formed by the preferential K+ dissolution at the membrane surface. The potentiometric response to a series of ammonium ions was in the following order; NH4+ > (CH3)NH3+ > (CH3)(2)NH2+ much greater than (CH3)(3)NH+, (CH3)(4)N+, CH3CH2NH3+, (CH3CH2)(2)NH2+, (CH3CH2)(4)N+, (CH3)CNH3+, CH3C3H6NH3+, CH3C3H6NH2+CH3, (CH3)(2)CHCH2NH2+CH3, reflecting the size of the analyte ions. On the contrary, the response to divalent heavy metal cations (selectivity; Cu2+ > Pb2+ > Zn2+ > Mn2+ > Ni2+ > Cd2+) however seemed to be due to specific adsorption onto the solid surface rather than the uptake of relevant ions into the vacancies formed by the preferential K+ dissolution. The latter mechanism appeared to be similar to the one previously reported for conventional precipitated-based ISEs, such as CuS, CdS and AgX (X = Cl, Br and I).