In the rapid expansion of supercritical solutions (RESS), the time scale available for phase separation is shown to govern the morphology of precipitated polymers. When the solution cloud point curve (CPC) is crossed inside the RESS nozzle, precipitation occurs over microseconds, freezing the structures formed during early stages of phase separation. Polymers precipitated from chlorodifluoromethane under such conditions occur as 0.2-0.6-mum particles. When the CPC is crossed in the preheating zone upstream of the nozzle, precipitation occurs over tens of seconds. This allows the structures to grow, and polymers are precipitated experimentally as 1-13-mum particles and large aspect ratio fibers. The time scale explanation correctly accounts for the effects on polymer morphology of pressure, temperature, concentration, and molecular weight. Rapid precipitation in the nozzle also produces single-T(g) blends of poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) and suppresses crystallization of poly(L-lactic acid) (PLA). Slower precipitation upstream of the nozzle produces a PMMA/PEMA blend with two T(g)’s and a relatively more crystalline PLA. Thus, the dynamics of the RESS process can be tuned to control both the shape and the phase morphology of polymer precipitates.