The paper describes developmental apparatus for in situ determinations of membrane swelling and shows representative examples of the data that can be acquired. The apparatus principally comprises a linear inductive probe and electronic column gauge with an overall resolution of 0.1 mu m which was used in two configurations to assess the swelling propensity of polyacrylonitrile (PAN)/polydimethylsiloxane (PDMS) nanotiltration membranes in a range of alkane, aromatic and alcohol solvents. In the absence of an applied pressure on the membrane, experiments showed a maximum expansion for the PDMS layer of 169% using an n-heptane solvent whose solubility parameter (delta) was close to that of PDMS. With more polar solvents falling in the range delta = 23.6-29.2 MPa0.5, swelling of the PDMS was much reduced (< 14%) and comparable shrinkage of the PAN support layer was also observed. If a mechanical pressure was applied to the membrane then swelling was reduced. For example, with a xylene solvent, over the pressure range 0-10 bar a progressive decline in membrane swelling from 118 to 50% was observed. At 20 bar swelling was further reduced to 33%. When xylene or heptane solvent was mixed with methanol, ethanol or propanol, reduced swelling of the PDMS layer occurred as the concentration of alcohol increased. The extent of swelling was closely related to the value of the mixture solubility parameter (delta(mixture)), where a higher value of led to less swelling. The results of the swelling experiments are compared to some of the authors previously published results for crossflow nanotiltration and shown to support the salient features. (c) 2005 Elsevier B.V. All rights reserved.