Solid-State H-1-NMR measurements of T1 relaxation times performed on polycarbonate-poly(ethyleneterephthalate) (PC-PET) blends point out the presence of two separate domains with apparent dimensions of about 80 nm. The variation of PET domain relaxation time with the increase of PC content is explained in terms of an interface in which parts of the PC molecules are finely dispersed into the PET matrix. Relaxation parameters and compositions match very well an equation that quantitatively describes a three-phase model formed by two domains separated by an interface of mixed components. Micrographs obtained by transmission electron microscopy (TEM) clearly reveal the presence of two separate domains with a phase inversion at 40/60 wt % composition. PET domains, although larger than expected from NMR analysis, are characterized by a dispersion of small PC particles that are considered responsible for the observed diffusion of magnetization from PET to PC domains. The partial miscibility seems to be physical in nature rather than due to transesterification processes between the components, as stem from H-1-NMR spectra in solution of PET and PC-PET blends. T1 relaxation times measured in the same way on totally immiscible PC-PA-6 blends, support, by contrast, the NMR interpretation of PC-PET results. The mechanical properties of PC-PET blends exhibit ductile behavior throughout the entire range of composition. This indicates that PC and PET are mechanically compatible. This is also in agreement with the isothermal crystallization data for PET at various compositions of PC-PET. These results are in agreement with the existence of a partial miscibility between PET and PC.