Metastable polymer blends of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) are formed by spraying polymer solutions into liquid and supercritical fluid CO2. Because of the rapid mass transfer between the CO2-phase and the solution phase, the blends are trapped in a metastable state before they can phase separate. A transition from a metastable blend with a single glass transition temperature, T-g in the form of particles, to a phase-separated 100 mu m fibre is observed with an increase in polymer concentration from 3.0 to 9.0wt%. This transition is related to the calculated concentration for the dilute to semi-dilute transition, C*, of PC/SAN in THF and the characteristic times for polymer-polymer and polymer-solvent phase separation. PC and SAN do not form a blend at 35 degrees C because of plasticization by CO2. They also do not form a metastable blend when 1.0mm droplets are precipitated into a liquid antisolvent, heptane.