The simultaneous spinning of two polymer solutions by using a triple orifice spinneret presents several benefits such as the possibility of forming each membrane layer by different concepts. However, besides the complexity of the phenomena involved in hollow fiber formation, mass transfer between the polymer solutions requires further investigation. In the present work, the simultaneous spinning of two different polymer solutions to form the top and support layers of the membrane, respectively, was investigated. Polyetherimide and polyethersulfone were used as base polymers. A polymer solution composed of Lewis acid:base complex (adipic acid-Lewis acid, and N-methyl-2-pyrrolidone-Lewis base), and another composed of a macromolecular additive (polyvinylpyrrolidone) were selected in order to form the support layer. Different polymer solutions composed of a volatile component (tetrahydrofuran) were used for top layer. Water and N-methyl-2-pyrrolidone were used at different compositions for the bore liquid, whereas pure water was used for the external precipitation bath. The distance between spinneret and precipitation bath was varied. The membranes were characterized by scanning electron microscopy. Cloud-point experiments were also carried out for characterizing the miscibility region of the polymer solutions. The parameters that allow keeping the stability in the region between the polymer solutions before interpenetration of the solutions seemed to be of greater importance for layers adhesion. According to this, the bore liquid composition seemed to provide an important contribution to fulfill such a requirement. (C) 2003 Elsevier B.V. All rights reserved.