Samples of PC-PET blends over the entire composition range were drawn at 160 degrees C to high extensions, 2.1-5.8, to study the mechanical reinforcement and the molecular structure development upon deformation. Elastic modulus E’ increases with extension ratio for all compositions and temperatures. Blends with 25 and 40 wt% of PC show higher E’ at low temperature than pure PET blends do. Crystallinity increases with extension ratio and is relatively smaller with increasing PC content. The influence of the reversible alpha to beta crystal form transformation was also studied. The second moment of the orientation function f for both crystal forms increases to high values > 0.9 at relatively low extensions. f decreases with PC content for alpha crystals but decreases for beta crystals. The alpha fraction is high for PET and decreases with PC content and extension ratio in the blends, Strain recovery experiments show that the alpha to beta transformation is also elastic in nature at high extension ratios and that the reinforcing effect in high PET content blends is not due to the alpha/beta ratio.