Poly(butylene succinate-ran-butylene azelate) random copolyesters (PBS-ran PBAz) were studied using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). These copolymers are characterized by being isodimorphic and by displaying a eutectic behavior. Depending on their composition, the PBS-rich phase, the PBAz-rich phase, or both phases can crystallize. The complex development of the amorphous phase, especially at compositions around the eutectic point was demonstrated by BDS results. The comonomer fraction included in the amorphous phase was quantified from the relaxation strength of the dielectric beta relaxations, and thus the crystalline fraction can be calculated. The good agreement between the values determined from BDS and the crystallinity obtained from DSC experiments demonstrates that the degree of comonomer exclusion in these random copolymers during crystallization is far larger than comonomer inclusion. However, the glass transition temperatures determined from the dielectric alpha relaxation behavior are lower in most of the copolymers than those measured by DSC, whereas they are about the same in the homopolymers. This result is probably caused by a significant amount of amorphous phase in the copolymers that is not influenced by crystallinity. Furthermore, the crystallization behavior of these copolymers at different cooling protocols showed that the alpha relaxation depends not only on the crystalline fraction but also on the details of the crystallization process in these complex multiphasic copolymers.