Abstract: Phosphate ion-sensitive composition and ion-selective electrode are disclosed incorporating an ionophore having the formula:R.sub.1 -X (a)orX-Z-X (b)wherein R.sub.1 is H or a hydrophobic substituent, Z is a spacing unit and X is represented by the formula: ##STR1## wherein n=0 or 1 and R.sub.2 and R'.sub.2 are independently selected from the group consisting of CH.sub.2, C(O) or C(S).