Microporous ethylene chlorotrifluoroethylene (ECTFE) membranes are expected to become industrially useful. Its solvent resistance is important in applications involving solvent microfiltration, organic synthesis, and membrane solvent extraction (MSX). Recent characterizations of microporous.ECTFE membrane after exposure to different liquid media and radiation, indicated that pure tri-n-octylamine (TOA) does have some effect. However, it is used in MSX with diluents, e.g., xylene. Therefore, many material and porous-structure characterization techniques and dead-end microfiltration were employed to study solvent-treatment effects on ECTFE membranes exposed to ethanol, xylene, xylene80/TOA20, and pure TOA. Membrane-surface roughness of virgin, ethanol-soaked, and TOA-soaked membranes indicated TOA-soaked membranes were the roughest, followed by ethanol soaked and virgin ones. Bubble-point-pressure based maximum pore diameters (d(max)) of solvent-treated membranes were: d(max),(TOA) > d(max,Xylene/TOA) > d(max,Xylene) > d(max,Ethanol) > d(max,Virgin). In dead-end microfiltration, fouling mechanisms behaved differently for virgin and TOA-soaked membranes; filtrate particle size distributions agreed well with estimated pore sizes. Additional characterizations indicated the limited effect of ethanol and xylene; however, TOA-soaked membrane behaved differently. In FTIR and Raman spectra, TOA introduced extra peaks indicating contributions from C-H stretching and deformation. Raman spectra of xylene80/TOA20-soaked membrane were a combination of those of xylene and TOA.