This thesis represents a study of the performance of potentiometnc ion sensors incorporating modified calixarene compounds as the lonophores. Three different groups of modified calixarenes were studied, namely tetramenc, hexamenc and oxa calixarenes and formed the basis of Na+, Cs+ and K+ sensors respectively. Tetramenc calix[4]arenes containing ketone functionalities methyl ketone (If), tbutyl ketone (Ig) and adamantyl ketone (Ih) and ester functionalities methyl ester (Ic), ethyl ester (Id) and «-butyl ester (Ie) were incorporated into mini electrode and macro electrode sensors with a conventional liquid filling solution. A further two denvatives containing the monoacid tnmethylester (Ij) and the monoacid tnethylester (Ik) were also assessed for their lonophonc potential and compared to their tetra methylester and tetra ethylester counterparts. All of the ester compounds and the methyl ketone denvative were found to exhibit excellent selectivities for sodium ions over the alkali and alkaline earth metals. An effort was made throughout the research to identify the functionalities in these new class of complexing ligands which are most likely to induce and enhance selective lonbinding. The caesium selective electrodes were based on hexamenc calix[6]arenes ligands (Ila) and (lib). Both ligands contained an ethylester moiety in the substituted lower nm position with an unsubstituted (Ha) and p-t-butyl substituted (lib) para position. The potassium selective electrodes are based on some monooxa and dioxa calix[4]arenes and on the ethylester p-f-butyl calix[5]arene. The oxa calixarenes contain an additional methoxy spacer unit in the macrocyclic nng and hence have cavity sizes intermediate between that of the tetramenc and hexamenc structures. The electrodes based on these ligands showed varying degrees of selectivity and stability but are unlikely to provide viable individual ion sensors for potassium. The final section of the thesis is devoted to solid state sensors. Electrodes were constructed by contacting the PVC liquid membrane either directly onto a platinum substrate or indirectly where the contact between the PVC and the platinum was provided by a layer of conducting polymer polypyrrole (PPy). The calix[4]arene involved was the tetramenc ethylester denvative (Id). Good functioning electrodes were produced and the PPy layer was found to enhance the stability of the contact.