Rapid-expansion-of-supercritical-solution (RESS) experiments were performed for solutions of cellulose triacetate [CTA; weight-average molecular weight 145,700, polydisperity = 2.07] in ethyl acetate [EA] over a range of concentrations and conditions of polymer-solvent phase behavior. Solid-solubility experiments were carried out beforehand and identified hot and compressed liquid EA at approximately 175degreesC as a good solvent for CTA. Cloud-point measurements were then used to locate the region of liquid-liquid equilibrium (LLE) for this system. The RESS results indicate that the phase state of the pre-expansion mixture determines the product size, and the overall concentration of the pre-expansion mixture determines the product morphology. However, we have also discovered a new constraint: for the production of well-formed fibers and particles, the rapid-expansion path must include penetration into a region of LLE, which must exist over a sufficient pressure range so that a separate, polymer-rich liquid phase has time to develop before the onset of vapor-liquid equilibrium (VLE). If rapid expansions are carried out at temperatures near or below the lower critical end point, the expansion path leads directly into a region of VLE, and a disruptive vapor expansion occurs within the continuous liquid phase. In this case, hollow particles, hollow structures, and even foams are produced. By the proper choice of operating conditions, we were able to produce continuous CTA fiber from a 5 wt % solution in EA at 250degreesC and 69 bar. (C) 2004 Wiley Periodicals, Inc.