This work was aimed at studying the effects of incorporation of epoxidized soybean oil (ESO) in a standard bisphenol A-type epoxy resin (EP) cured by anhydride hardener. The EP/ESO ratio was set for 100/0, 75/25, 50/50, 25/75, and 0/100 (wt%/wt%). The investigations performed covered the curing, rheology (gelling), thermomechanical (TMA), and thermogravimetric analysis (TGA) of the EP/ESO compositions. The results showed that the dilution of EP with ESO was accompanied with marked changes in the curing, gelling behavior, and final properties. Differential scanning calorimetry revealed that the crosslinking of EP/ESO 50/50 occurred in two steps. This has been considered for the cure schedule set. The gel time of EP/ESO, determined at T = 100, 120, 140 degrees C, respectively, increased with increasing ESO content. The activation energy of gelling increased with increasing ESO content. The glass transition temperature decreased with increasing ESO content. The samples were transparent that was traced to the presence of domains smaller in size than the wavelength of the visible light based on atomic force microscopy inspection. According to TMA, the coefficient of thermal expansion in the glassy state increased with increasing ESO content but was independent of the latter in the rubbery stage. TGA indicated that with increasing ESO content the thermal degradation started earlier and the char yield decreased. The Ozawa, Flynn, and Wall (OFW) approach was adapted to TGA tests to calculate the activation energy of thermal degradation. The activation energy depended on the ESO content of the EP/ESO blends and also on their actual decomposition stage. The latter means a limitation for the OFW approach. POLYM. ENG. SCI., 54:747-755, 2014. (c) 2013 Society of Plastics Engineers