), were prepared and the electrode characteristics were tested. The conditioned NH 4 + -ISEs (E1) based on L 1 with TEHP as a plasticising solvent mediator gave best results with near-Nernstian slope of 53.9 mV/decade of activity, detection limit of 10 −4.9 M, and enhanced selectivity coefficients for the NH 4 + ion with respect to an interfering K + ion (log K pot NH 4 + ,K + = -1.84). This result was compared to other ammonium ionophores reported previously, for example, that of nonactin (log K pot NH4 + ,K + = -0.92). The proposed electrode showed no significant potential changes in the range of 3.0 < pH < 9.0.