Recently more attention has been paid to the mesomorphic phase of isotactic polypropylene (iPP), not the least because better calorimetric techniques and instruments had become available, enabling studies at high cooling rate. In addition to the scientific interest sparked by the availability of novel differential scanning calorimetry (DSC) equipment enabling cooling rates up to 3.000 K/s (FSC, fast scanning chip calorimetry), the mesomorphic phase also plays an important role in high-speed conversion processes for iPP like cast-film extrusion, metal coating and cable insulation. The presence of this phase can be either positive, e.g. for transparency and toughness in case of films, or negative in terms of long-term stability and post-crystallization. Recent investigations have shown that the reduction of crystallization rate by increasing chain disturbance (stereodefects or comonomers) shifts the growth transitions from the á-crystalline to the mesmorphic phase to lower cooling rates. An extensive cast film study was performed with several iPP homopolymers of different isotacticity and an ethylene-propylene (EP) random copolymer at two different film thickness levels (50 and 200 µm). Melt and chill roll temperature were varied systematically together with the processing speed. The produced films were characterized by standard tests and wide-angle X-ray scattering (WAXS). The parameter variation resulted in a significant difference of crystallinity, mechanical and optical properties together with the behavior on subsequent annealing and further processing. At higher effective cooling rates the mesomorphic phase dominated the films, leading to higher transparency and lower modulus. In parallel, the polymers were studied by FSC technique at 0,1 to 1.000 K/s, seeking to use the results for a better understanding of iPP film casting. It must however be concluded that the competition between flow induced crystallization and quenching effects defines the film properties.