Rapid expansion from supercritical solution (RESS) of a crystalline fluoropolymer, poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) or poly(TA-N), in carbon dioxide produces submicron to several micron sized particles and fibers. The understanding of the RESS mechanism has been clarified by careful design of experimental variables and procedures. The concentration of the poly(TA-N)/CO2 solution was held constant (at 0.5 and 2.0 wt %), the solution cloud point curves were obtained, the pre-expansion temperature was varied above and below the cloud point, and the length to diameter (L/D) ratio of the nozzle was varied from 8.5 to 508. The morphology is explained in terms of the location of phase separation within the expansion nozzle. The. L/D is the most influential variable for achieving a transition from particles or fibers. In most cases, manipulation of the solution concentration and the pre-expansion temperature did not produce this transition but did have a large effect on the sizes of the particles and fibers. These results are an important step in demonstrating CO2-based spray processes which do not require any volatile organic solvents.