The construction, performance, and application of mercury (II) - selective PVC membrane electrodes based on 7,8-dinitro-4,11-dioxa-22,32-dithia-1,14- diazapentacyclo [22.214.171.124(5.10). 10(16.21).0(26.31)] tetratriaconta-5,7,9,16,18,20,26,28,30-nonane (cryptand I) and 7-nitro-4,11-dioxa-22,32-dithia-1,14-diazapentacyclo [126.96.36.199(5.10).10.(16.21).0(26.31)] tetratriaconta-5,7,9,16,18,20,26,28,30-nonane (cryptand II) as novel cryptand derivatives are reported. An optimum membrane composition of 2% ionophore, 68.3% plasticizer, 29.7% poly(vinyl chloride) (PVC), and 75% mol lipophilic anionic additive with respect to the ionophore is found for both of the electrodes. The influences of plasticizer, internal filling solution and pH on the potentiometric response of the electrodes were investigated. The electrodes exhibit twice-Nernstian response for Hg (II) ion over a wide concentration range within solutions of pH 2.0 - 3.0. The proposed electrodes show good selectivity for mercury (II) ion in comparison with alkali, alkaline earth, transition and heavy metal ions and especially silver(I) ions. The electrodes have a response time of about 15 - 20 s and they can be used for at least I month without any divergence. The practical applications of the proposed cryptand I and II based mercury (II) - selective electrodes were observed by using them as indicator electrodes in potentiometric titrations of mercury (II) ions with EDTA. Hg (II) ISE based on cryptand II appeared to be more suitable than Hg (II) ISE based on cryptand I for potentiometric titrations.