The pH-dependent distribution of the indicator dye tetrabromophenolphthalein ethyl ester (TBPE, chromoionophore VIII) between plasticized polymeric phase and aqueous solution is studied. The commonly utilized algorithms for partition properties calculation are evaluated experimentally in terms of their suitability for the ionized species. The physical-chemical characteristics of TBPE that determine its applicability for use in chemical sensors, namely, the lipophilicity and acidity constant are estimated in the hydrophobic polymeric phase for the first time. The distribution coefficients of the neutral (protonated) and ionized (deprotonated) forms of TBPE are determined in plasticized PVC/water biphase system as 7.2 and 3.8, respectively (logk, ionic strength 1 M, 25 °C). The acidity constant of TBPE in plasticized PVC membrane, pK_a^PVC−DOS, is estimated spectrophotometrically as 6.1 ± 0.2 log units. The results can be used for developing optical chemical sensors based on TBPE (chromoionophore VIII), in particular for predicting the sensor lifetime, optimization of the sensor operating range and measuring conditions.