Novel biobased epoxy resins were prepared from two fatty acid derivatives; epoxidized 10-undecenoyl triglyceride and epoxidized methyl 3,4,5-tris(10-undecenoyloxy)benzoate, with 4,4-diaminodiphenylmethane as a crosslinking agent. The flame retardancy of these epoxy resins was improved by the addition of 10-[2', 5'-bis(9-oxiranyl-nonayloxy)phenyll-9,10-dihydro-9-oxa-10-phosphaphenant hrene-10- oxide and by crosslinking with a phosphorus-containing curing agent, bis(m-aminophenyl)methylphosphine oxide. The thermal, thermomechanical, and flame-retardant properties of the cured materials were measured with differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, and the limiting oxygen index. (c) 2006 Wiley Periodicals, Inc.