RESS experiments were performed on the semicrystalline fluoropolymer poly(heptadecafluorodecyl acrylate) [poly(HDFDA); M-w = 254 000, M-w/M-n = 2.95] from solutions in carbon dioxide, with both concentration and degree of saturation being varied in a systematic manner. Phase-behavior measurements were carried out beforehand to locate the liquid-liquid equilibrium phase boundaries and thus establish the degree of saturation for a given concentration and preexpansion temperature and pressure. Concentrations of 0.5-5 wt % polymer were rapidly expanded through a short orifice (LID = 5.08) at preexpansion temperatures and pressures corresponding to unsaturated, saturated, and supersaturated solutions. The results indicate that the size of the polymer product is controlled by the degree of saturation (S); however, the morphology of the product is controlled primarily by the concentration, not by S. Only at some intermediate concentration, a threshold concentration where fiber formation first starts to occur, does S play a significant role. Thus, for example, micron-sized particles are produced by rapidly expanding a,supersaturated solution of 0.5 wt % poly(HDFDA) in CO2, whereas continuous, submicron fibers are produced by the rapid expansion of an unsaturated solution containing 5 wt % of the polymer.