Three new calixarene Tl+ ionophores have been utilized in Tl+ ion-selective electrodes (ISEs) yielding Nernstian response in the concentration range of 10−2-10−6 M TlNO3 with a non-optimized filling solution in a conventional liquid contact ISE configuration. The complex formation constants (log βIL) for two of the calixarene derivatives with thallium(I) [i.e., 6.44 and 5.85] were measured using the sandwich membrane technique, with the other ionophore immeasurable due to eventual precipitation of the ionophore during these long-term experiments. Furthermore, the unbiased selectivity coefficients for these ionophores displayed excellent selectivity against Zn2+, Ca2+, Ba2+, Cu2+, Cd2+ and Al3+ with moderate selectivity against Pb2+, Li+, Na+, H+, K+, NH4+ and Cs+, noting that silver was the only significant interferent with these calixarene-based ionophores. When optimizing the filling solution in a liquid contact ISE, it was possible to achieve a lower limit of detection of approximately 8 nM according to the IUPAC definition. Last, the new ionophores were also evaluated in four solid-contact (SC) designs leading to Nernstian response, with the best response noted with a SC electrode utilizing a gold substrate, a poly(3-octylthiophene) (POT) ion-to-electron transducer and a poly(methyl methacrylate)/poly(decylmethacrylate) [PMMA-PDMA] co-polymer membrane. This electrode exhibited a slope of 58.4 mV decade−1 and a lower detection limit of 30.2 nM. Due to the presence of an undesirable water layer and/or leaching of redox mediator from the graphite redox buffered SC, a coated wire electrode on gold and graphite redox buffered SC yielded grossly inferior detection limits against the polypyrrole/PVC SC and POT/PMMA-PDMA SC ISEs that did not display signs of a water layer or leaching of SC ingredients into the membrane.