Three sets of new block copolymers PUI with increasing soft segment lengths were obtained by a method which allows the control of the material structure by a step-by-step polycondensation : polyetherurethaneimides (PEG-MDI-AP), polyetherureaimides (JFA ED-MDI-AP) and polyesterurethaneimides (PCL-MDI-AP) with soft block molecular weights in the range of 300-2000. At 30 degrees C, all the PUI exhibited selectivity towards ethanol during the pervaporation of EtOH/ethyl tert-butylether mixtures close to the azeotropic composition and the PV fluxes were related to the polymer structure in a quantitative way. They were shown to vary exponentially with the soft segment weight fraction in the case of PUL polyethers whereas linear relationships described the partial and total fluxes obtained with PUI polyesters. The second part of the investigations dealt with the influence of the soft segment length on the PV features of PUI at higher temperatures. Despite one particular polymer leading to a significant deviation from the usual behaviour, the PV mass transfer of both permeants in all the PUI investigated obeys an Arrhenius type law in the temperature range of 30-60 degrees C. The activation energies could be related to the material structure qualitatively, showing an important decrease of these parameters for the polymers with the longest soft segments.