Bisphenol A dicyanate (BADCy) was modified by diglycidyl ether of bisphenol A epoxy resins with different molecular weights [E20 (weight-average molecular weight = 1000) and E51 (weight-average molecular weight = 400)] to investigate the effects of the epoxy molecular weight on the properties of the modified systems. The reactions were monitored with differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy, and the results showed that more pentacyclic oxazolidinone rings were formed in BADCy/E51 than in BADCy/E20 with the same epoxy resin weight content. DSC showed that BADCy/E20 had a lower curing temperature than BADCy/ E51 because of the higher concentration of hydroxyl groups (-OH) in E20. Thermal, moisture absorption, and mechanical testing showed that E51-modified BADCy performed better because of its lower molecular weight. (c) 2006 Wiley Periodicals, Inc.