Combined experimental and modeling approaches were performed in order to investigate the influence of formulation and process parameters on mass transfers during VIPS process, using the water/N-methyl-pyrrolidone (NMP)/poly(ether imide) (PEI) system. The experiments were conducted using a thick polymer solution at increasing polymer concentrations for various operating conditions. The global water intake rate in the bulk solution was determined by gravimetric measurements (global), and in situ measurements were conducted by near-infrared spectroscopy at three points in the solution. In parallel, a fully predictive model was developed for predicting mass-transfer phenomena involved during the VIPS process. The comparison between experimental data and numerical predictions exhibited a good agreement for moderate polymer concentration, but for higher polymer concentrations, the model overestimated the nonsolvent-transfer rate. This result was explained by the aggregation process of the polymer chains due to water intake. The numerical predictions were improved by modifying the average hole-free volume expression. (C) 2012 American Institute of Chemical Engineers AIChE J, 59: 671-686, 2013