A new approach to reinforce and toughen isotactic polypropylene (iPP) with improved processability is evaluated. The concept involves using a crystallizable solvent that, at process temperatures melts, is miscible with the polymer thereby reducing its process viscosity. As the polymer cools, the solvent undergoes thermally induced phase separation (TIPS) to produce crystallites that increase the modulus of the solid through reinforcement and promote an increase in impact resistance by mechanisms similar to rubber-toughened materials. Tetrabromobisphenol-A (TBBPA) is introduced to iPP that forms a homogeneous mixture at elevated temperature and acts as a processing aid, but undergoes phase separation and subsequent crystallization upon cooling to form rigid particles which, in turn, acts as a toughening agent at room temperature. A phase diagram constructed with Flory-Huggins solution thermodynamics shows good agreement with the experimental results. The steady state shear viscosity decreases as TBBPA content increases for the mixtures in melt state, indicating improved processability. The decrease in viscosity enhances crystallization rate of iPP significantly, most likely due to increased diffusivity, while the structure of iPP crystals remain unchanged. Tensile tests show that as TBBPA content increases (up to 15 wt%), the yield stress decreases while elongation at break increases. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3564-3576, 2009