Blend membranes of poly(bis phenol-A-ether sulfone) (PSF) and poly(n-vinyl pyrrolidone) (PVP) in ratios (90:10 to 10:90; with increments of 10) were prepared via solution casting technique. The membranes were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the blend is immiscible having the major phase of the blend homogeneously dispersed in the continuum of the minor phase indicating the presence of "reverse phase morphology" (RPM) by SEM analysis of the dissolution treated membranes. The interaction parameter chi(PVP/PSF) was calculated and the ternary phase diagram with tentative spinodal along with the super-imposed experimental cloud points has been illustrated. The symmetric spherical geometry of the dispersed phase was explained theoretically with the help of the Flory Huggins Theory. The performance of these membranes as rate controlling membranes in controlled release applications was studied by coating them on paracetamol tablets and the effect of the ratio of PVP in the blend membrane on the rate of drug release was monitored. Models of mass transfer employing Fickian principles at constant temperature and pressure were elucidated to support the experimental findings. The predicted models were found to be in excellent agreement with the experimental release profiles. (C) 2003 Elsevier B.V. All rights reserved.