A kinetic model of formation of hyperbranched polymers by step polyaddition of a BA(f) monomer including cyclization was developed. Reactions between acyclic-acyclic and acyclic-cyclic molecules and cyclization within acyclic molecules were considered. For cyclization within isomers of an x-mer, a bridging function between linear and spherical structures was used. Fractions of cyclic x-mers are predicted to increase with increasing degree of polymerization a: and overall conversion of B groups. High fractions of cyclics were found in poly[2,2-bis(hydroxymethyl)propanoic acid] (poly(dimethylolpropionic acid)) by mass spectroscopy (ESI/FT MS) in a reasonable agreement with the model.