Hydroxyl terminated alkyds synthesized from castor oil, glycerol, and different dibasic acids were used to develop uralkyds and their interpenetrating polymer networks (IPNs) with polybutyl methacrylate (PBMA). Glass transition temperature measurements gave the evidence of interpenetration. The IPNs were characterized for their physicomechanical properties and their phase morphology was studied by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). As the concentration of the uralkyd in IPNs increases, a gradual increase in elongation, density, and swelling with a consequent decrease in the hardness were observed for all IPNs. Swelling is relatively more prominent in methyl ethyl ketone (MEK) and toluene than in water. From SEM it was observed that IPNs of PBMA-uralkyd containing phthalic anhydride (UA-P) as an acid part showed greater compatibility than those containing dimethyl terephthalate (UA-D). From thermogravimetric analysis (TGA) no significant change was observed in the degradation behavior of the IPNs. (C) 2000 John Wiley & Sons, Inc.