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• Plasticization of CA is carried out with octanoic acid, ethyl and butyl lactate. • OA, EL, BL decrease the mechanical relaxation of the CA. • BL supresses totally the crystalline region of the CA. • A representation of the behavior of CA with the increasing plasticizer is proposed. • Two relaxations appear explained by the concept of CA and plasticizer rich phases. This work brings together thermo-mechanical and structural information for plasticized cellulose acetate (CA) by lactates and octanoic acid. CA are processed with plasticizer due to their high Tg and their strong H-bonding network. We prepared CA / plasticizer blends by corotative twin screw extruder and by solvent casting methods. The study of the different relaxations and of the glassy zone modulus was performed by dynamic mechanical analysis (DMA). The miscibility range of cellulose acetate blends were identified by the analysis of the tan δ. Depending on the composition of the system, one or two transitions are noted, this last result indicates the presence of a phase rich in CA and another in plasticizer. To connect this information to crystallinity and molecular organization, X-ray diffraction analyses were carried out. The disappearance of crystallinity allows the plasticization of previously inaccessible zones, causing a glassy modulus drop of more than 1000 MPa. [ABSTRACT FROM AUTHOR]