An aliphatic-aromatic pehenolic hyperbranched starting material (HBP-OH) with phenolic end groups was 85% functionalized by a Mitsunobu reaction with oxetane groups (OXT-HBP). This new hyperbranched polyester was used, at a concentration of 5-20 wt %, as an additive for the cationic photopolymerization of a commercial oxetane-based resin, 4,4'-bis[(3-ethyl-3-ethyl-3-oxetanyl) methoxymethyl]biphenyl (OXBP). HBP-OXT acted as a multifunctional crosslinker, copolymerizing with the oxetane ring of the CXBP resin, reacting through chain transfer with the remaining phenolic OH groups, or doing both. The result was an increase in the glass-transition temperature due to the increase in the crosslinking density. An increase in the weight residue at a high temperature was found in the presence of HBP-OXT and was attributed to the presence of phenolic groups, which are commonly used as antioxidant additives. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 293-299, 2005.