A nondestructive technique is presented for in-situ analysis of solvent induced membrane swelling. The technique is based on spectroscopic ellipsometry (SE) and allows simultaneous measurement of membrane film thickness and solvent sorption. The attainable experimental conditions are only limited by the test cell (≤473.15 K, ≤20 MPa) and by far exceed those encountered in typical solvent resistant nanofiltration (SRNF) applications. The influence of cross-linker concentration on swelling of thin films and bulk samples of poly(dimethyl siloxane) (PDMS) is resolved and interaction parameters according to Flory–Huggins and Flory–Rehner theories are calculated. It is found that due to confinement on the support for the thin supported films a correction for elastic deformation of the network is necessary. In the pressurized swollen film the thickness and refractive index are found to be independent of pressure in the range of 0.1–10 MPa suggesting that the molar volumes of the penetrant in the liquid and sorbed phases are not significantly different. When a pressure difference over the membrane is applied and the solvent is allowed to permeate, a progressing reduction in thickness of the membrane is observed with increasing upstream pressure. The derived concentrations of n-hexane at the interface between thin film and support, at the permeate side, are in excellent agreement with values calculated using the solution–diffusion model. This implies that in-situ spectroscopic ellipsometry allows quantification of the contribution of solution–diffusion to mixed mode transport, for instance occurring simultaneously with pore-flow and transport through defects, in composite membranes.