EtOH/ETBE azeotropic mixture was separated by prevaporation through films of polyurethaneimides (PUI), i.e., alternating block copolymers varying in their structure only by their oligomeric soft blocks. Pervaporation performances strongly depend on the flexible segments : fluxes on the molecular weight and selectivity towards ethanol on the chemical structure in the decreasing order : PEG, PCL, PCD, PTMG, PPG. In order to correlate selectivity with segment polarity, a new solvatochromic polarity probe that was well soluble in PUI was synthesized. Using 13 solvents covering a wide polarity range, Vis lambda(max) of the photochromic indicator open form was linearly correlated with the E(T)(30) polarity scale. Ey illumination of the dissolved dye, PUI polarity was investigated in relation to-soft segment nature and size. The very high PUI polarity values and their splitting for highest size segments were assigned to preferential probe solvation by interblock urethane junctions combined with phase segregation. Polarity values consistent with the chemical structure of the flexible segments were provided by similar measurements on suitably end-capped precursors of these segments and were then linearly correlated with the related PUI pervaporation selectivity.