Blends of polycarbonate (PC) and poly(alkylene terephthalate) (PAT) such as poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) were investigated. It was learned that processes of phase separation in blends consisting of PC and PAT can cause variations in properties of both the amorphous and crystalline phases. In PC/PBT blends the DSC technique did not detect crystalline portion of PBT with its concentrations up to 20 wt%. For PBT = 40 wt%, it forms a continuous phase, and blend's crystallinity is close to the additive values. The glass transition temperature (T-g) shifts to the lower temperature region. The relaxation spectrometry revealed strong adhesion between phases in the blends over the temperature range from the completion of beta-transition to T-gPAT. This interaction becomes weaker between T-gPAT and T-gPC. Temperature-dependent variations in the molecular mobility and interphases interactions in the blends affect their impact strength. Over the temperature range where interphases interactions occur and the two components are in the glassy state, the blend is not impact resistant. Over the temperature range between T-gPAT and T-gPC the blends become impact-resistant materials. This is because energy of crack propagation in the PAT amorphous phase - being in a high-elastic state - dissipates. It is postulated that the effect of improving the impact strength of PC/PAT blends, which was found for temperatures between the glass transition temperatures of the mixed components, is also valid for other binary blends.