The mechanisms of tri-dimensional polyesterifications in bulk at 180 degrees C of ternary mixtures of o-phthalic anhydride (P)+ azelaic acid (Z)+ trimethylolpropane (T), as well as the distributions of acid branches on the potential crosslinking T units, have been studied by liquid C-13 n.m.r. before (sol) and after (gel) gelation. The critical time for gelation is found to decrease when Z increases, and in all cases the actual critical conversions are larger than those calculated by the classical statistical Flory-Stockmayer relation. Transesterification reactions actually occur during polyesterification. Before as well as after gelation, the distributions of P and Z in diesters and triesters of T follow two slightly different Bernouillian statistics because of the better reactivity of Z than P.