Amorphous nylon, poly( iminoisophthaloyliminomethylene-1,3-cyclohexylenemethylene) (NyBI) and poly(ethylene oxide) (PEO)-segmented NyBI (PEO-NyBI) membranes were prepared by a phase-inversion method using water/dimethyl sulfoxide (DMSO) mixtures as coagulants. The influence of the PEO segment and coagulant compositions on the morphology of the membranes was investigated. The cloud-point curves in the polymer/DMSO/water ternary system showed that PEO-NyBI and NyBI had the same coagulation processes, that is, instantaneous liquid-liquid phase separation occurred, resulting in a fingerlike structure in the cross section of the membranes. The membrane morphologies observed under a scanning electron microscope (SEM) agreed with the prediction. The PEO segment had little or no effect on the membrane morphologies which were prepared in the coagulants with a low DMSO concentration, and it promoted the change of the phase-separation style from the instantaneous to the delayed one in the case of the DMSO-rich coagulant. The PEO segment, however, significantly influenced the ultrafiltration rate. Additionally, the relationship between the surface composition of the PEO-NyBI membrane and the coagulation condition was also investigated by use of electron spectroscopy for chemical analysis (ESCA) and static secondary ion mass spectrometry( SSIMS). A small enrichment of the PEO segment at the top surface of the membranes was observed with the increase of the DMSO concentration in the coagulant.