Herein we report the synthesis, optical, and electrochemical properties of a calix[4]arene-~ubstituted polythiophene which demonstrates ion selective voltammetric, chromic, fluorescent, and resistive responses. The ionochromic response of this polythiophene on exposure to Na+ shows an increased effective conjugation length of the polymer backbone. Despite this, Na+ induces a large positive shift in the potential at which the polymer is oxidized (>+lo0 mV) commensurate with a large decrease in conductivity ('99%). Although the calix[4]arenesubstituted polythiophene exhibits no changes in the W v i s spectrum and only minimal changes in the voltammetric responses on exposure to Li+ or K+, there are large decreases in relative conductivities (69% and 47%, respectively). Thus, although the sensory properties of this polymer are expressed via several measurable entities, the ionoresistive response is clearly the most sensitive. This sensitivity originates from the cooperative nature of carrier transport in a CP and is thus inherent in chemoresistive CPs.