The gelation and crystallization processes in 5 wt% solutions of syndiotactic polystyrene in acetophenone and acetophenone + carbon dioxide fluid mixtures were studied in a variable-volume view-cell system over a pressure range from 7 to 50 MPa for CO2 concentrations up to 40 wt%. The solution in pure acetophenone displayed only a sol-gel (gelation) phase boundary. Solutions containing 25 wt% CO2 displayed both a sol-gel and a liquid-liquid phase boundary. Solutions containing 40% CO2 displayed instead a fluid-solid (crystallization) and a liquid-liquid phase boundary. The consequences of crossing the gelation, crystallization or the liquid-liquid demixing boundaries first on the eventual morphology and the crystal structures that develop were investigated. It is shown that when the gelation boundary is crossed first, the resulting crystal structure is the delta form. When the L-L phase boundary is crossed first, polymer-rich phase leads to a mixture of delta + beta crystal forms while only the delta form is found in the polymer-lean phase from solutions with 25wt% CO2. Crossing the crystallization boundary first in solutions with 40 wt% CO2, leads to the form with no presence of the delta crystal form. However, when the liquid-liquid phase boundary is crossed first, the polymer-rich phase forms only the beta crystal form and the polymer-lean phase leads to a mixed crystal structure containing the beta and delta forms. Formation of the different crystal structures are verified by XRD, FTIR and SEM characterizations and are rationalized with a phase diagram describing the paths the system follows as a result of a change in pressure and/or the temperature. (C) 2008 Elsevier B.V. All rights reserved.