The kinetics of the polycondensation of A(2) and B-3 monomers leading to hyperbranched polyesters was studied by NMR spectroscopy and compared with theoretical predictions. Combinations of two different A2 monomers, the aromatic terephthaloyl chloride (TC1, AD and the aliphatic adipic acid (AA, AA respectively, with 1,1,1-tris(4-trimethylsiloxyphenyl)ethane (TMS-THPE, B-3) and 1,1,1-tris(4-hydroxyphenyl)ethane (THPE, BA respectively, were studied in different molar ratios of the monomers. On the basis of a complete H-1 and C-13 NMR signal assignment for the structural units of the two resulting hyperbranched polymers their relative concentration as a function of conversion was determined. In the case of TC1/TMS-THPE, the degree of branching just before gelation increased with increasing A2 content. A further increase of DB could be obtained by sequential addition of the A(2) + B-3 monomer mixture. Since the kinetic data for that system did not describe the development of structural units expected for equal and independent reactivity of the functional groups (ideal case), the relative rate constants of the basic reaction steps were determined by simulation to fit best the experimental data. A decrease of the reactivity of the second A function after reaction of the first and a small increase of the reactivity of B functions after reactions of other B groups in the molecule were found. The kinetic data for the less reactive system AA/THPE, however, followed the development expected for an ideal polymerization even though very long reaction times had to be applied at high reaction temperature which favored side reactions.