A fourth generation hyperbranched polyester polyol based on bis-MPA and a PP50 core molecule (Boltorn H40) as well as its fractions obtained by preparative SEC were characterized with respect to composition, molar mass, and structure using NMR, SEC-MALS, ESI-MS, and DSC techniques. The content of dendritic units and the degree of branching (DB) are significantly lower than expected for a random polymerization, thus indicating that the hydroxyl groups in the linear repeat units are less reactive than those in terminal repeat units. The main side reaction in the synthesis of Boltorn H40 is deactivation of the bis-MPA carboxyl groups leading to structures without a core molecule. This side reaction influences mainly M-n, and to a lesser degree the M-w value of Boltorn H40. Furthermore, some of the hydroxyl groups react to ether groups. Etherification as a side reaction is of minor importance since the content of ether groups is below 1%. The obtained results indicate that despite deactivation of carboxyl groups of bis-MPA, the use of the PP50 core molecule in the polycondensation of bis-MPA permits control of the molar mass and polydispersity index, whereas the DB is not influenced. The Mw of Boltorn H40 is comparable to, and M. is significantly lower than, the theoretical molar mass of dendritic poly(bis-MPA) with a PP50 core molecule.